Cloud water characterization system

ABSTRACT

A system and method for providing a statistical measure of the size of liquid water droplets in a cloud, as well as a system and method for the detection and/or measurement of the presence of a cloud, liquid water content in the cloud and ice water content in the cloud, among other parameters.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was partially made with U.S. Government support underContract No. NAS3-02162. Accordingly, the U.S. Government may havecertain rights in this invention.

BACKGROUND OF THE INVENTION

The present invention is generally directed to aircraft icing sensors,and in particular, to a system and methodology for providing among otherthings as set forth herein, a measure of a water droplet size in a cloudcontaining liquid water, ice water or both.

Aircraft icing is a serious safety problem for general aviation and somecommuter transport airplanes. There has been tremendous growth in thecommuter aviation industry in the last few years. When compared to heavytransport category aircraft, these types of aircraft generally operateat lower altitudes and consequently spend a greater proportion of theirtime operating in icing conditions.

Airframe icing severity depends on the liquid water content, dropletsize and degree of glaciation of the cloud. Liquid water content (LWC)is normally expressed in grams/cubic meter and is a measure of theunfrozen water content of the cloud. Droplets occur in nature not in asingle size but in a distribution of sizes. The median volume diameter(MVD) of the droplet distribution has historically been used as a bulkmeasurement of droplet spectrum size. For example, FAA aircraft andengine certification criteria are expressed in terms of a combination ofLWC and MVD.

The droplet size plays a critical role in the way that ice accumulateson airframe and engine surfaces. In general the larger the MVD the moreproblems the accumulated ice will cause. Small droplets tend to freezeimmediately on impact whereas larger droplets flow along the structuralsurfaces before freezing. If the drops have a very large MVD, i.e.greater than 50 microns, they may run past the anti-ice/deice systemsand freeze in critical, unprotected places on the airframe. These arethe so-called supercooled large drop (SLD) cases. SLD encounters are ofparticular interest for aviation safety.

Cloud total water content (TWC) includes both unfrozen droplets (liquidwater) and frozen water in the form of ice crystals. The degree ofglaciation in the cloud can be determined by comparing the liquid watercontent to the total water content.

In general, airframe and engine icing depend primarily on liquid watercontent because the unfrozen, but super-cooled droplets, freeze uponimpact whereas already frozen ice crystals will not. However, there aresome very important exceptions. In some particular cases of temperatureand airspeed, ice crystals can melt upon impact and then refreeze. Thiscan cause severe problems for both airframes and engines.

In the past several years there have been a number of fatal commuteraircraft accidents attributed to severe icing conditions having SLD.Though SLD was thought to occur infrequently, the significant increasein commuter aircraft traffic has raised a concern that the chances ofencountering this icing condition may be far greater than previouslythought. At the present time aircraft ice protection systems are notrequired to provide protection against SLD. Therefore a simple devicethat could provide warning that SLD conditions are present would behighly desirable.

Cylindrical and wire like sensors for measuring LWC are known. However,the heating for such known sensors use external windings to heat thesensor. The external windings are quite delicate giving these sensors avery short life due to damage from encounters with typical atmosphericconcentrations of ice crystals. Generally speaking sensors usingexternal heater windings are problematic because the windings createsmall “pockets” that trap ice crystals causing a significant fraction ofice crystals to be captured and evaporated. The evaporation of ice froma liquid water sensor therefore gives false measurements or indicationsof liquid water when in the presence of ice crystals.

Other prior art LWC sensors have used an internal, switched indirectlyheated, cylindrical element to collect super cooled liquid water thatfreezes onto the cylinder. The cylinder is oscillated at its naturalmechanical resonant frequency (approximately 40 kHz). As ice builds upthe frequency decreases to a preset value at which time a heating cycleis initiated to clear the ice from the cylinder. The internal heatercycles on and off to remove the accumulated ice. The rate of cyclingprovided an indication of the amount of liquid water present.

However, it is believed that deficiencies exist in these prior artarrangements, such as the ability to reject unwanted ice crystals whenmeasuring LWC. Tests have shown that the present invention provides abetter than ten times improvement over previous LWC sensors in rejectingunwanted ice crystals, due to the shape and surface construction of thesensors.

The development of portions of the present invention has been carriedout under a NASA Small Business Innovative Research (SBIR) contractbetween the NASA Glenn Research Facility and Science EngineeringAssociates, Inc. (SEA) as part of NASA's Aviation Safety Programinitiative to significantly reduce aircraft accidents. Part of thisprogram, is to establish the frequency of occurrence of SLD conditions.Once this frequency of occurrence is established, it would be expectedthat agencies like the Federal Aviation Administration (FAA) could usethis information to affect changes in current aircraft icingcertification regulations. This in turn would lead to changes in thedesign of aircraft ice protection systems, potentially to protectagainst severe icing conditions.

Present research activities aimed at establishing this frequency ofoccurrence have been limited to a handful of research organizationsusing heavily instrumented research aircraft (e.g. the NASA GlennResearch Centers' Twin Otter Icing Research Aircraft and the NRCConvair-580 aircraft). Though effective, this approach is limited inscope and somewhat biased because the research flights are specificallydirected into areas having the highest probability of severe icingconditions. Therefore a more extensive approach, such as that offered byinstrumenting a large number of commercial/military aircraft, would beof great value. The key to this approach is the development of a small,reliable, low power integrated icing instrument, which has notpreviously been available.

It is believed that the present invention, in addition to providingneeded capabilities in a research environment, will also help providethe needed protection against icing in private and commercial aircraftand overcomes the deficiencies set forth above and achieves theobjectives set forth herein.

SUMMARY AND OBJECTIVES OF THE PRESENT INVENTION

It is thus an objective of the present invention to overcome thedeficiencies existing in the state of the art.

In particular, an objective of the present invention is to provide aplurality of sensors that are each designed, calibrated, co-located andoriented within a small common shroud, for the calculation of cloudparameters previously only possible by using multiple instruments onresearch aircraft.

Yet another objective of the present invention is to provide astatistical measure of the size of liquid water droplets in the cloud,commonly called the MVD (Median Volumetric Diameter).

Still another objective of the present invention is to overcome thedifficulties found in the prior art of duplicating cloud conditions, andMVD in particular, in wind tunnel testing.

Yet another objective of the present invention is to provide a systemand method for measurement and detection of cloud water conditions thatis lightweight and suitable for use on non-research aircraft.

Further objectives of the present invention are to provide a system andmethod for the detection and/or measurement of the presence of a cloud,LWC, Ice Water Content (IWC), and SLD parameters.

Still other objectives and advantages of the invention will in part beobvious and will in part be apparent from the specification.

The invention accordingly comprises the features of construction,combination of elements, arrangement of parts and sequence of stepswhich will be exemplified in the construction, illustration anddescription hereinafter set forth, and the scope of the invention willbe indicated in the claims.

Therefore, and generally speaking, in one embodiment, the presentinvention is directed to a system for determining a measure of cloudwater droplet size based on sensor cooling by cloud water and storedsensor power ratio and droplet size data, the system comprising a firstand second temperature controlled co-located self heated sensors, thecooling of the sensors being substantially independent of ice water; aself heated compensation sensor oriented to be substantially independentof cloud water cooling; a variable power source coupled to each firstand second sensor for keeping each sensor at a temperature and eachhaving a measurable output power; a variable power source coupled to thecompensation sensor for keeping the sensor at a temperature and having ameasurable output power; and a control unit for correcting the measuredpower to each of the first and second sensors to reflect only heat lossdue to cloud water by subtracting from the measured power an adjustedcompensation sensor measured value; and forming a ratio of correctedpower of each of the first and second sensors to identify a liquid waterdroplet size based on the stored power ratio and droplet size data.

In another embodiment, the present invention is directed to a system formeasuring water droplet size in a cloud, wherein the system comprises asensor head comprising a first heated sensor having a resistancecharacteristic that is temperature dependent, wherein power loss fromthe sensor is at least essentially not affected by the presence of icewater in an airflow thereacross; a second heated sensor having a secondresistance characteristic that is temperature dependent, wherein powerloss from the second sensor is at least essentially not affected by thepresence of ice water in the airflow thereacross; a heated compensationsensor being positioned to be responsive to the airflow thereacross andhaving a power loss at least essentially not affected by the presence ofliquid water or ice water in the airflow thereacross; a respectivetemperature controller associated with each of the first sensor, secondsensor and compensation sensor, each of which for maintaining thetemperature of the respective sensors at respective temperature valuesusing its temperature dependent resistance characteristic, wherein, foreach of the first and second sensors, the respective controllermaintains the sensor at the temperature value by adjusting a power levelfed to the sensor; calculates the total power required to maintain thesensor at the temperature value when the sensor is exposed to an airflowcomprising at least one of liquid water and ice water; and a controlunit for receiving power levels from each of the first sensor and secondsensor; subtracting, from each total power calculation, a dry powercomponent determined from the power required to maintain thecompensation sensor at its temperature value in the presence of theairflow comprising at least liquid water, thereby arriving at a wetpower value attributable to the incremental power associated withmaintaining the respective first and second sensors at the respectivetemperature values in the presence of at least liquid water in theairflow; and for forming a ratio of the wet power value of the firstsensor to the wet power value of the second sensor to obtain anestimated value of the measure of water droplet size in the cloud from apredetermined calibrated relationship of wet power ratio to waterdroplet sizes.

In yet another embodiment, the present invention is directed to a systemfor determining liquid water content in a cloud, wherein the systemcomprises a sensor head comprising a first heated sensor having a firstresistance characteristic that is temperature dependent, wherein thefirst sensor is at least essentially not affected by the presence of icewater in an airflow thereacross; a second heated sensor having a secondresistance characteristic that is temperature dependent, wherein thesecond sensor is at least essentially not affected by the presence ofice water in the airflow thereacross; a heated compensation sensor beingpositioned to be responsive to the airflow thereacross and at leastessentially not affected by the presence of liquid water or ice water inthe airflow thereacross; a respective temperature controller associatedwith each of the first sensor, second sensor and compensation sensor,each of which for maintaining the temperature of the respective sensorsat respective temperature values using its temperature dependentresistance characteristic, wherein, for each of the first and secondsensors the respective controller maintains the sensor at thetemperature value by adjusting a power level fed to the sensor;calculates the total power required to maintain the sensor at thetemperature value when the sensor is exposed to an airflow comprising atleast liquid water; and a control unit for receiving power levels fromeach of the first sensor and second sensor; subtracting, from each totalpower calculation, a dry power component determined from the powerrequired to maintain the compensation sensor at its temperature value inthe presence of the airflow thereby arriving at a wet power valueattributable to the incremental power associated with maintaining therespective first and second sensors at the respective temperature valuesin the presence of at least liquid water in the airflow; and calculatinga liquid water content for the first sensor and sensor wet power valuesand selecting the higher LWC as the desired liquid water content.

In still another embodiment of the present invention, a system isprovided for determining liquid water content in a cloud, the liquidwater having a determined droplet size, wherein the system comprises asensor head comprising a first heated sensor having a first resistancecharacteristic that is temperature dependent, wherein the first sensoris at least essentially not affected by the presence of ice water in anairflow thereacross; a second heated sensor having a second resistancecharacteristic that is temperature dependent, wherein the second sensoris at least essentially not affected by the presence of ice water in theairflow thereacross; a heated compensation sensor being positioned to beresponsive to the airflow thereacross and at least essentially notaffected by the presence of liquid water or ice water in the airflowthereacross; a respective temperature controller associated with each ofthe first sensor, second sensor and compensation sensors, each of whichfor maintaining the temperature of the respective sensors at respectivetemperature values using its temperature dependent resistancecharacteristic, wherein, for each of the first and second sensors therespective controller maintains the sensor at the temperature value byadjusting a power level fed to the sensor; calculates the total powerrequired to maintain the sensor at the temperature value when the sensoris exposed to an airflow comprising at least liquid water; and a controlunit for receiving power levels from each of the first sensor and secondsensor and having a stored droplet size verses liquid water contentcorrection curve for each first and second sensor; subtracting, fromeach total power calculation, a dry power component determined from thepower required to maintain the compensation sensor at its temperaturevalue in the presence of the airflow thereby arriving at a wet powervalue attributable to the incremental power associated with maintainingthe respective first and second sensors at the respective temperaturevalues in the presence of liquid water in the airflow; and calculating aliquid water content for the first sensor and second sensor wet powervalues, selects the higher of the calculated liquid water contentvalues, and corrects the higher of the two values using a droplet sizeverses liquid water content correction curve.

In yet another embodiment, a system is provided for determining thepresence of ice water in an airflow, wherein the system comprises asensor head comprising a first heated sensor having a first resistancecharacteristic that is temperature dependent, wherein heat the fromfirst sensor is at least essentially not affected by the presence of icewater in the airflow thereacross; a second heated sensor having a secondresistance characteristic that is temperature dependent, wherein heatfrom the second sensor is at least essentially not affected by thepresence of ice water in the airflow thereacross; a third heated sensorhaving a third resistance characteristic that is temperature dependent,wherein heat loss from the third sensor is affected by a presence of icewater in the airflow thereacross; a heated compensation sensor beingpositioned to be responsive to the airflow thereacross and having a heatloss least essentially not affected by the presence of liquid water orice water in the airflow thereacross; a respective temperaturecontroller associated with each of the first sensor, second sensor,third sensor and compensation sensors, each of which for maintaining thetemperature of the respective sensors at respective temperature valuesusing its temperature dependent resistance characteristic, wherein, foreach of the first, second and third sensors, the respective controllermaintains the sensor at the temperature value by adjusting a power levelfed to the sensor; calculates the total power required to maintain thesensor at the temperature value when the sensor is exposed to an airflowcomprising at least ice water; and a control unit for receiving powerlevels from each of the first sensor and second sensor; subtracting,from each total power calculation, a dry power component determined fromthe power required to maintain the compensation sensor at itstemperature value in the presence of the airflow comprising at least icewater, thereby arriving at a wet power value attributable to theincremental power associated with maintaining the respective first,second and third sensors at the respective temperature values in thepresence of at least ice water in the airflow; and calculating a watercontent measurement for the first sensor, second sensor and third sensorwet power values, selecting a higher water content value from the firstand second sensor and subtracting the higher value from the watercontent value of the third sensor to indicate a presence of ice water.

In still another embodiment, a system is provided for warning of cloudwater, the system comprising a first temperature controlled self heatedsensor, a variable power source for the first sensor having a measurableoutput power responsive to changes in cloud water; a processor foraveraging the measured output power of the power source and subtractinga substantially instantaneous measure of the output power of the powersource to determine fluctuations around the average; and comparing thefluctuations to a threshold indicative of the presence of cloud water.

In still further embodiments, the system for determining a measure ofcloud water droplet size based on sensor cooling by cloud water andstored sensor power ratio and droplet size data, may comprise means forkeeping each of the first and second sensors at a temperature and eachhaving a measurable output power; for keeping the compensation sensor ata temperature and having a measurable output power; correcting themeasured power to each of the first and second sensors to reflect onlyheat loss due to cloud water by subtracting from the measured power anadjusted compensation sensor measured value; and forming a ratio ofcorrected power of each of the first and second sensors to identify aliquid water droplet size based on the stored power ratio and dropletsize data.

Similarly, in yet another embodiment, the system for measuring waterdroplet size in a cloud may comprise means associated with each of thefirst sensor, second sensor and compensation sensor, for maintaining thetemperature of the respective sensors at respective temperature valuesusing its temperature dependent resistance characteristic, wherein, foreach of the first and second sensors, the means maintains the sensor atthe temperature value by adjusting a power level fed to the sensor andcalculates the total power required to maintain the sensor at thetemperature value when the sensor is exposed to an airflow comprising atleast one of liquid water and ice water; and means for receiving powerlevels from each of the first sensor and second sensor; subtracting,from each total power calculation, a dry power component determined fromthe power required to maintain the compensation sensor at itstemperature value in the presence of the airflow comprising at leastliquid water, thereby arriving at a wet power value attributable to theincremental power associated with maintaining the respective first andsecond sensors at the respective temperature values in the presence ofat least liquid water in the airflow; and for forming a ratio of the wetpower value of the first sensor to the wet power value of the secondsensor to obtain an estimated value of the measure of water droplet sizein the cloud from a predetermined calibrated relationship of wet powerratio to water droplet sizes.

Likewise, the system for determining liquid water content in a cloud maycomprise means associated with each of the first sensor, second sensorand compensation sensor, for maintaining the temperature of therespective sensors at respective temperature values using itstemperature dependent resistance characteristic, wherein, for each ofthe first and second sensors the sensor is maintained at the temperaturevalue by adjusting a power level fed to the sensor; and the total powerrequired to maintain the sensor at the temperature value when the sensoris exposed to an airflow comprising at least liquid water is calculated;and means for receiving power levels from each of the first sensor andsecond sensor; subtracting, from each total power calculation, a drypower component determined from the power required to maintain thecompensation sensor at its temperature value in the presence of theairflow thereby arriving at a wet power value attributable to theincremental power associated with maintaining the respective first andsecond sensors at the respective temperature values in the presence ofat least liquid water in the airflow; and calculating a liquid watercontent for the first sensor and sensor wet power values and selectingthe higher LWC as the desired liquid water content.

Moreover, the system for determining liquid water content in a cloud,the liquid water having a determined droplet size, may comprise meansassociated with each of the first sensor, second sensor and compensationsensors, each of which for maintaining the temperature of the respectivesensors at respective temperature values using its temperature dependentresistance characteristic, wherein, for each of the first and secondsensors the means maintains the sensor at the temperature value byadjusting a power level fed to the sensor and calculates the total powerrequired to maintain the sensor at the temperature value when the sensoris exposed to an airflow comprising at least liquid water; and means forreceiving power levels from each of the first sensor and second sensorand having a stored droplet size verses liquid water content correctioncurve for each first and second sensor; subtracting, from each totalpower calculation, a dry power component determined from the powerrequired to maintain the compensation sensor at its temperature value inthe presence of the airflow thereby arriving at a wet power valueattributable to the incremental power associated with maintaining therespective first and second sensors at the respective temperature valuesin the presence of liquid water in the airflow; and calculating a liquidwater content for the first sensor and second sensor wet power values,selects the higher of the calculated liquid water content values, andcorrects the higher of the two values using a droplet size verses liquidwater content correction curve.

Still further, the system for determining the presence of ice water inan airflow may comprise means associated with each of the first sensor,second sensor, third sensor and compensation sensors, each of which formaintaining the temperature of the respective sensors at respectivetemperature values using its temperature dependent resistancecharacteristic, wherein, for each of the first, second and thirdsensors, the means maintains the sensor at the temperature value byadjusting a power level fed to the sensor and calculates the total powerrequired to maintain the sensor at the temperature value when the sensoris exposed to an airflow comprising at least ice water; and means forreceiving power levels from each of the first sensor and second sensor;subtracting, from each total power calculation, a dry power componentdetermined from the power required to maintain the compensation sensorat its temperature value in the presence of the airflow comprising atleast ice water, thereby arriving at a wet power value attributable tothe incremental power associated with maintaining the respective first,second and third sensors at the respective temperature values in thepresence of at least ice water in the airflow; and calculating a watercontent measurement for the first sensor, second sensor and third sensorwet power values, selecting a higher water content value from the firstand second sensor and subtracting the higher value from the watercontent value of the third sensor to indicate a presence of ice water.

The present invention is also directed to a plurality of methodologies.For example, in accordance with a first embodiment, a method fordetermining a measure of cloud water droplet size based on sensorcooling by cloud water and stored sensor power ratio and droplet sizedata, utilizing a system comprising a first and second temperaturecontrolled co-located self heated sensors, the cooling of the sensorsbeing substantially independent of ice water, a self heated compensationsensor oriented to be substantially independent of cloud water cooling;a variable power source coupled to each first and second sensor forkeeping each sensor at a temperature and each having a measurable outputpower; and a variable power source coupled to the compensation sensorfor keeping the sensor at a temperature and having a measurable outputpower, wherein the method comprises the steps of correcting the measuredpower to each of the first and second sensors to reflect only heat lossdue to cloud water by subtracting from the measured power an adjustedcompensation sensor measured value; and forming a ratio of correctedpower of each of the first and second sensors to identify a liquid waterdroplet size based on the stored power ratio and droplet size data.

In yet another embodiment, the present invention is directed to a methodof measuring water droplet size in an airflow, utilizing a systemcomprising a sensor head comprising a first heated sensor having aresistance characteristic that is temperature dependent, wherein powerloss from the sensor is at least essentially not affected by thepresence of ice water in the airflow thereacross; a second heated sensorhaving a second resistance characteristic that is temperature dependent,wherein power loss from the second sensor is at least essentially notaffected by the presence of ice water in the airflow thereacross; aheated compensation sensor being positioned to be responsive to theairflow thereacross and having a power loss at least essentially notaffected by the presence of liquid water or ice water in the airflowthereacross, wherein the method comprises the steps of maintaining thetemperature of the respective sensors at respective temperature valuesusing its temperature dependent resistance characteristic; and wherein,for each of the first and second sensors, the method comprises the stepsof maintaining the sensor at the temperature value by adjusting a powerlevel fed to the sensor and calculating the total power required tomaintain the sensor at the temperature value when the sensor is exposedto an airflow comprising at least one of liquid water and ice water;wherein the method further comprises the steps of receiving power levelsfrom each of the first sensor and second sensor; subtracting, from eachtotal power calculation, a dry power component determined from the powerrequired to maintain the compensation sensor at its temperature value inthe presence of the airflow comprising at least liquid water, therebyarriving at a wet power value attributable to the incremental powerassociated with maintaining the respective first and second sensors atthe respective temperature values in the presence of at least liquidwater in the airflow; and forming a ratio of the wet power value of thefirst sensor to the wet power value of the second sensor to obtain anestimated value of the measure of water droplet size in the cloud from apredetermined calibrated relationship of wet power ratio to waterdroplet sizes.

In still yet another embodiment, a method is provided for determiningliquid water content in an airflow, utilizing a system comprising asensor head comprising a first heated sensor having a first resistancecharacteristic that is temperature dependent, wherein the first sensoris at least essentially not affected by the presence of ice water in anairflow thereacross; a second heated sensor having a second resistancecharacteristic that is temperature dependent, wherein the second sensoris at least essentially not affected by the presence of ice water in theairflow thereacross; a heated compensation sensor being positioned to beresponsive to the airflow thereacross and at least essentially notaffected by the presence of liquid water or ice water in the airflowthereacross; wherein the method comprises the steps of maintaining thetemperature of the respective sensors at respective temperature valuesusing its temperature dependent resistance characteristic, maintainingthe sensors at the temperature value by adjusting a power level fed tothe sensor; calculating the total power required to maintain the sensorat the temperature value when the sensor is exposed to an airflowcomprising at least liquid water; receiving power levels from each ofthe first sensor and second sensor; subtracting, from each total powercalculation, a dry power component determined from the power required tomaintain the compensation sensor at its temperature value in thepresence of the airflow thereby arriving at a wet power valueattributable to the incremental power associated with maintaining therespective first and second sensors at the respective temperature valuesin the presence of at least liquid water in the airflow; and calculatinga liquid water content for the first sensor and sensor wet power valuesand selecting the higher LWC as the desired liquid water content.

Still in another embodiment, a method is provided for determining liquidwater content in an airflow, the liquid water having a determineddroplet size, utilizing a system comprising a sensor head comprising afirst heated sensor having a first resistance characteristic that istemperature dependent, wherein the first sensor is at least essentiallynot affected by the presence of ice water in an airflow thereacross; asecond heated sensor having a second resistance characteristic that istemperature dependent, wherein the second sensor is at least essentiallynot affected by the presence of ice water in the airflow thereacross; aheated compensation sensor being positioned to be responsive to theairflow thereacross and at least essentially not affected by thepresence of liquid water or ice water in the airflow thereacross;wherein the method comprises the steps of maintaining the temperature ofthe respective sensors at respective temperature values using itstemperature dependent resistance characteristic and, for each of thefirst and second sensors, maintaining maintains the sensor at thetemperature value by adjusting a power level fed to the sensor andcalculating the total power required to maintain the sensor at thetemperature value when the sensor is exposed to an airflow comprising atleast liquid water; wherein the method further comprises the steps ofreceiving power levels from each of the first sensor and second sensorand having a stored droplet size verses liquid water content correctioncurve for each first and second sensor; subtracting, from each totalpower calculation, a dry power component determined from the powerrequired to maintain the compensation sensor at its temperature value inthe presence of the airflow thereby arriving at a wet power valueattributable to the incremental power associated with maintaining therespective first and second sensors at the respective temperature valuesin the presence of liquid water in the airflow; and calculating a liquidwater content for the first sensor and second sensor wet power values,selects the higher of the calculated liquid water content values, andcorrects the higher of the two values using a droplet size verses liquidwater content correction curve.

In yet another embodiment, a method is provided for determining thepresence of ice water in an airflow, utilizing a system comprising asensor head comprising a first heated sensor having a first resistancecharacteristic that is temperature dependent, wherein heat the fromfirst sensor is at least essentially not affected by the presence of icewater in the airflow thereacross; a second heated sensor having a secondresistance characteristic that is temperature dependent, wherein heatfrom the second sensor is at least essentially not affected by thepresence of ice water in the airflow thereacross; a third heated sensorhaving a third resistance characteristic that is temperature dependent,wherein heat loss from the third sensor is affected by a presence of icewater in the airflow thereacross; a heated compensation sensor beingpositioned to be responsive to the airflow thereacross and having a heatloss least essentially not affected by the presence of liquid water orice water in the airflow thereacross; wherein the method comprises thesteps of maintaining the temperature of the respective sensors atrespective temperature values using its temperature dependent resistanceand wherein, for each of the first, second and third sensors,maintaining the sensor at the temperature value by adjusting a powerlevel fed to the sensor and calculating the total power required tomaintain the sensor at the temperature value when the sensor is exposedto an airflow comprising at least ice water; wherein the method furthercomprises the steps of receiving power levels from each of the firstsensor and second sensor; subtracting, from each total powercalculation, a dry power component determined from the power required tomaintain the compensation sensor at its temperature value in thepresence of the airflow comprising at least ice water, thereby arrivingat a wet power value attributable to the incremental power associatedwith maintaining the respective first, second and third sensors at therespective temperature values in the presence of at least ice water inthe airflow; and calculating a water content measurement for the firstsensor, second sensor and third sensor wet power values, selecting ahigher water content value from the first and second sensor andsubtracting the higher value from the water content value of the thirdsensor to indicate a presence of ice water.

In still another preferred embodiment of the present invention, a methodis provided for warning of cloud water using a first temperaturecontrolled self heated sensor and a variable power source for the firstsensor having a measurable output power responsive to changes in cloudwater, wherein the method comprises the steps of averaging the measuredoutput power of the power source; subtracting a substantiallyinstantaneous measure of the output power of the power source todetermine fluctuations around the average; and comparing thefluctuations to a threshold indicative of the presence of cloud water.

BRIEF DESCRIPTION OF THE DRAWINGS

The above set forth and other features of the invention are made moreapparent in the ensuing Description of the Preferred Embodiments whenread in conjunction with the attached Drawings, wherein:

FIG. 1 is a view of an overall system constructed in accordance with thepresent invention;

FIGS. 2A, 2B, 2C, 2D, 2E, 2F, and 2G are perspective, detail andcross-sectional views of a sense head constructed in accordance with thepresent invention;

FIGS. 3A, 3B, 3C and 3D are differing views of the various sensors usedin the present invention;

FIG. 4 is an exemplary temperature control circuit constructed inaccordance with the present invention for use in connection with each ofthe sensors set forth in FIGS. 3A, 3B, 3C and 3D;

FIG. 5 is a block diagram of a deice temperature control circuitconstructed in accordance with the present invention;

FIG. 6 is an exemplary resistance-temperature characteristic curve forone of the sensors of FIG. 3;

FIG. 7 relates dry power for a compensation sensor to dry power for amedium, a thin and a scoop sensor;

FIG. 8 relates a ratio of sensor wet power to MVD value based on windtunnel tests;

FIG. 9 is an exemplary correction curve to correct liquid water contentmeasured value based on MVD data;

FIG. 10 depicts a control unit constructed in accordance with theinvention; and

FIG. 11 depicts an alternate embodiment of a control unit in accordancewith the invention.

Identical reference numerals in the figures are intended to indicatelike parts, although not every feature in every figure may be called outwith a reference numeral.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As will be set forth in greater detail below, the present invention,generally speaking, uses the heat loss from a medium diameter sensor,thin diameter sensor and semi-cylindrical shaped scoop-like sensor thatare each designed, calibrated, co-located and oriented within a smallcommon shroud, for the calculation of cloud parameters previously onlypossible by using multiple instruments on research aircraft.

Among other things, the invention measures cloud ice water content(IWC), liquid water content (LWC) and total cloud water content (TWC),which is the sum of the cloud LWC and IWC, all of which are measured ingm/m³. In addition, the present invention provides a statistical measureof the size of liquid water droplets in the cloud (e.g. the MVD). Asused herein, the term MVD is defined as a droplet diameter that dividesthe total amount of cloud water in half; half the water volume will havedroplets with diameters larger than the MVD value and half of the watervolume will have droplet diameters smaller that the MVD value. MVD ismeasured in microns.

As is will be understood by one skilled in the art, there are a numberof measures of droplet size other than MVD that are useable with theinvention. These include: Average Drop Diameter, Median Drop Diameter,Mode Drop Diameter, Average Volume Diameter, Mode Volume Diameter andSauter Mean Diameter all of which can characterize an aggregate of dropswhere the individual drops come in various sizes. The particularcharacterization is substantially determined by the calibrationdevice(s)/method(s) used for the invention. The term MVD, which is theterm preferred by the FAA, is often used herein to characterize dropsize but its use is in no way intended to limit the invention.

As is known to those skilled in the art, cloud conditions, and MVD inparticular, are very difficult to duplicate consistently in differentwind tunnels or even in a single wind tunnel. This condition resultsfrom the nature of the hardware used to form cloud ice and water as wellas the instrumentation used to measure the parameters of the formedcloud. The present inventors have found, however, that the presentinvention provides excellent repeatability and allows its use as astandard for comparing wind tunnels as well as the accuracy andrepeatability needed to improve safety of flight in all aircraft.

The present invention uses the principle that the total electrical powerrequired to maintain a heated sensing element at a preset constanttemperature is substantially equal to the power lost due to interactionof the sensing element with the cloud environment. When the sensor ismoving relative to moisture laden air and heated to a temperature thatwill cause water in contact with a sensor to evaporate, then the totalheat loss can be expressed as the sum of the heat loss that would occurif there was no moisture being evaporated (dry power) and the heat lostfrom the sensor due to evaporation of liquid water or ice water present(as the case may be) on the sensor (wet power).

In addition, the invention uses the known principle that the heat lossfrom a specific sensor geometry can be used in an analysis of cloudwater to differentiate between water and ice as well as droplet sizes.The geometry of the sensor is important since it affects its ability to“capture” and or “hold” the water from a cloud long enough to causeevaporation.

The present inventors have also discovered in wind tunnel tests that theuse of a semi-cylindrical shaped self heated scoop-like sensor (scoopsensor) provides improved measurement of TWC as compared to prior artconical shaped TWC sensors. Likewise, the present inventors have foundthat a large fraction of the water present in ice crystals is lost whenthe ratio of the width to the depth of a scoop sensor is one half orless. By definition a half cylinder has a width to depth ratio ofexactly one half.

Through testing, the present inventors have found that adding additionaldepth in the form of straight sides, or extensions 174, as shown in FIG.3C, leading into the half cylinder greatly increases the amount of watermeasured from ice crystals. We have tested scoop sensors with a width todepth ratio of 0.5 to 4.0 and have found that deeper tends to be betterwith respect to ice water measurement, but the extra heated surface ofthe deeper ratio elements requires additional dry power, which requireslarger power supplies and their attendant disadvantages. A compromisebetween power and measurement accuracy occurs for a range of depth towidth ratios from between about 1.0 to about 2.0.

The loss of water from ice crystals is due to the fact that the icearrives inside the scoop in solid form which cannot spread out quickly,like a liquid droplet, until it is melted. This takes additional time.If the ice crystal and the liquid water derived from the ice crystal arenot retained in the scoop, then the accuracy of the measurement of theamount of ice is reduced.

In addition, the straight sides or extensions 174 on the basic halfcylinder establish circulation currents inside the scoop that tend tokeep ice and water from escaping from scoop 116. A second purpose of thestraight sides 174 is to help prevent the loss of ice crystals by directbouncing out of scoop 116 and back into the air stream.

An advantage using directly heated sensors is the increased intimacy ofcontact with the ice crystal in the case of the scoop sensor which helpsto impart heat to the crystal as rapidly as possible. Likewise with themedium and thin sensors, discussed below, the self heated configurationprovides improved evaporation efficiency for water droplets and allowssmooth surface conditions which helps discriminate against ice.

Also, as indicated above, testing has determined that there is a betterthan ten (10) times improvement over prior art LWC sensors in rejectingunwanted ice crystals due to the use of a medium and thin sensor havinga substantially smooth external surface of less than about 2.5 μm.Likewise we have found that the response time of each of the sensorswith their associated temperature controller power supply is less than0.01 seconds thus allowing the identification of pockets of cloud waterduring flight.

The present invention determines measures of LWC from the wet power lostfrom a medium and thin sensor and MVD by using a calibrated ratio of wetpower lost from the medium sensor to wet power lost from the thinsensor.

We have also discovered in wind tunnel tests that the preferred sensortemperature control systems and sensors, having low thermal mass, areresponsive to short term “bursts” of ice water or liquid water andallows early detection of cloud water.

The present invention provides two temperature control systems. Thefirst system is used to maintain each sensor at a known temperature sothat the wet and dry power values discussed above can be determined. Thesecond system controls the average temperature of the shroud to preventice build up on the sensor and minimize errors that may occur due toundesirable heat flow paths not related to the evaporation of water ordetermination of dry power. The heated shroud provides the additionalbenefit of operation even at zero airspeed.

Turning now to the figures in particular, reference is first made toFIG. 1, which illustrates an overall view of the system of the presentinvention, generally indicated at 5. System 5 comprises a sensor head,generally indicated at 10, a temperature controller generally indicatedat 20 and a control unit generally indicated at 30.

As illustrated in FIGS. 2A-2C, sensor head 10 includes a sensor module110 having four sensors; a medium sensor 112, thin sensor 114, scoopsensor 116, and compensation sensor 118. Sensor head 10 also includesheated shroud 120, a heated strut 130 and a mounting surface 18 formounting the sensor head 10 to an aircraft surface or wind tunnel (notshown). Sensor head 10 is preferably mounted with strut 130 in ahorizontal position for receiving air stream 104.

Temperature controller 20 includes individually controlled power sourcesthat power each sensor located in sensor module 110 and keep each of thesensors at a predetermined temperature which is preferably 140 degreesC. The strut 130 and shroud 120 include individual heaters (not shown inFIG. 1) that are powered by a single deice temperature control system308 shown in FIG. 5 that is also located in temperature controller 20.

The temperature of each sensor 114, 116, 112, and 118 is determined bymeasurement of each sensor's electrical resistance and holding thatresistance substantially constant by varying the amount of electricalpower passed through the sensor. The shroud and strut temperatures arecontrolled using a thermistor (not shown in FIG. 1) to feed back aresistance value from which temperature is readily calculated. Thethermistor 140 is located at the interface between the sensor module 110and the strut, as shown in FIG. 2F.

Sensor Details

Turning to FIGS. 2A and 2B in particular and as mentioned above, sensormodule 110 houses the four self heated sensors, namely scoop sensor 116,medium sensor 112, thin sensor 114 and compensation sensor 118. Thescoop sensor 116, medium sensor 112, and thin sensor 114 are orientedsubstantially normal to the direction of airflow and parallel to eachother. These sensors have a projected area in the flow of air ofapproximately their width times their height. The compensation sensor118, being oriented parallel to the airflow 104, has a projected areainto the airflow of approximately equal to its cross sectional area

Front, top (cross-sectional) and side (cross-sectional) views of sensorhead 10 are shown in FIGS. 2C-2G. Strut 130 assists to keep sensor head10 away from the mounting surface and fairing 134 provides a transitionfrom the sense module 110 to strut 130. The relative locations of thesensors 114, 116 and 112 and compensation sensor 118 are shown in thevarious figures of FIGS. 2D and 2E. In the preferred embodiment, sensors112 and 114 are mounted forward of the scoop sensor 116, which is itselfmounted forward of compensation sensor 118. This preferred layoutreduces errors due to liquid water bouncing off scoop sensor 116 fromstriking either the medium sensor 112 or thin sensor 114. Further,having scoop sensor 116 forward of compensation sensor 118 introducesturbulence in the vicinity of compensation sensor 118, thus improvingits linearity as a function of airspeed.

Small protrusion 122 in FIG. 2E is a small nickel plated copper pinpreferably about 0.063 inch dia×0.150 inch high (1.6 mm dia×3.81 mmhigh). Because the fwd mounting post for compensation sensor 118 ispreferably electrically insulated from both the inner shroud 126 and theouter shroud 128, it is not sufficiently deiced for all conditions bythe heated shroud.

Under very cold conditions ice can build up on the leading edge of themounting post for compensation sensor 118 and change the airflow aroundthe compensation sensor 118 resulting in a poor estimate of dry power.The purpose of protrusion 122 is to intercept and warm stray water thatpasses scoop sensor 116 and keep it from freezing onto the frontmounting post of the compensation element 118.

Compensation sensor 118 is mounted parallel to the airflow and thereforeexposed only to the cooling effects of the general flow and not tocooling from liquid water or ice water. Because the power required tomaintain the compensation sensor 118 at a given temperature in moisturefree air is proportional to only the amount of airflow in airstream 104,this power value is used to determine the wet power for each sensorwhich is the incremental power needed to evaporate water on the mediumsensor 112, the thin sensor 114, and the scoop sensor 116.

Because the geometries and locations of the medium sensor, thin sensor,scoop sensor and compensation sensors are different, correction curvesrelating dry power values as measured by the compensation sensor to drypowers for the medium sensor 112, thin sensor 114 and scoop sensor 116is required, the preferred embodiment being illustrated in FIG. 7.

FIGS. 3A-3C, in conjunction with Table 1 as set forth below, gives thepreferred mechanical and electrical parameters of thin sensor 114 (shownin FIG. 3A), medium sensor 112 (shown in FIG. 3B), scoop sensor 116(shown in FIG. 3C) and the compensation sensor 118 (shown in FIG. 3D).

Scoop sensor 116 is preferably substantially ½ of a directly heatedcylinder (split lengthwise) having flattened extensions giving anoverall width to depth ratio of about 1.0 and an overall length ofapproximately 24 mm, dimensions of which are illustrated in the FIG. 3C.

Scoop sensor 116 includes end caps 172 which are welded to both ends ofthe scoop sensor 116. End caps 172 include holes for receiving a neckeddown section of copper connector 166 for soldering to a shroud mountedpin 167. Shroud pin 167 is soldered to shroud 120 forming a groundconnection for scoop sensor 116. Medium sensor 112 receives neck downcopper connector 162 and neck down connector 168 receives thin sensor114 to make ground connections by soldering to shroud mounting pins 167in a manner similar to that described for scoop sensor 116. Copperconnectors 162, 166 and 168 are preferably silver soldered to each ofthe sensors. A similar arrangement (not shown) provides a groundconnection for compensation sensor 118 at 138 in FIG. 2G. Each ofconnection points 167 and 138 provide substantially rigid single endedsupports for each of the sensor to accommodate thermal expansion of thesensors 112, 114, 116 and 118.

Medium sensor 114 is preferably a cylinder so that the majority of thepower dissipated by the sensor takes place on its surface in directcontact with any water on the sensor. The thin sensor 114 is preferablya solid wire of stainless steel having the preferred dimensions given inFIG. 3A.

Surface roughness may be added to the inside of scoop sensor 116 in theform of ridges, convolutions, or indentations, which tend to increasethe retention of water melted from ice crystals and therefore give moreopportunity for evaporation. Alternatively, bonding a rough materialhaving a high electrical resistance and low thermal resistance, such asdiamond chips or silicon carbine chips, may be used to increase thesurface roughness. Testing indicates that a surface roughness of greaterthan about approximately 2.5 μm peak to peak for scoop sensor 116 andless than approximately 2.5 μm peak to peak for medium sensor 112 andthin sensor 114 are preferred.

Alternate embodiments of the scoop sensor 116 include shapes havingopen-sided rectangular, triangular, trapezoidal or square-likecross-sections through their long axis. Alternate embodiments of themedium sensor 112 and thin sensor 114 include shapes having rectangular,triangular, trapezoidal or square-like cross-sections through their longaxis.

FIGS. 2A-2G illustrates other features, including certain mountingfeatures, of the present invention, such as the insulation block,grounding and power source connections to each of the sensors.

Temperature Control Systems

The preferred embodiments of the invention provide for two basictemperature control system configurations, namely a control system shownin FIG. 4 that preferably maintains a sensor at 140° C. by controllingthe voltage across the sensor, and a control system shown in FIG. 5 thatapplies power to heaters 268, 264 and 266 to maintain the temperature ofthe shroud 120, fairing 134 and strut 130 at an average temperature ofpreferably 50° C. The basic difference between the temperaturecontrollers of FIGS. 4 and 5 is that in the sensor temperature controlsystem shown in FIG. 4, the resistance of the heated sensor is itself anindicator of its temperature whereas in the deice temperature controlsystem of FIG. 5, a thermistor provides a feedback indication of theshroud 120, strut 130 and fairing 134 temperature.

Sensor Control System

Four preferably similar sensor control circuits and software routinesare used for each of the sensors 114, 112, 116 and 118. The serialoutputs of the control circuits are combined in a multiplexer (notshown) and fed to control unit 30 or 3A for further analysis.

Preferably, the basic operation of all sensor temperature controlsystems are similar, so only system 208 for medium sensor 112 isdescribed herein, with the others being similarly constructed and thedescription thereof being omitted for purposes of brevity. However, itshould be understood that the sensor temperature control systems dodiffer in that each includes stored data that characterizes theparticular sensor in terms of its temperature-resistance characteristicin a manner suitable for temperature control and the calculation of wetand dry power. Characterization data for each sensor is given in Table 1and discussed further below.

TABLE 1 thin Medium Scoop Comp Sensor Sensor Sensor Sensor 114 112 116118 Sensor Dimensions Width (mm) 3.962 2.108 0.533 0.279 Length (mm)23.622 20.447 21.590 16.764 Area (mm²) 93.590 43.102 11.507 4.684 Sensorresistance Room Temp 53.965 32.145 24.185 76.170 as function of room  8557.435 34.370 30.160 93.510 and oil bath 100 58.345 34.865 31.815 98.270temperatures 115 59.195 35.430 33.515 103.025 130 60.035 35.935 35.245108.300 145 60.855 36.455 37.025 113.145 Characterization of Slope(dT/dR) 17.5783 28.6165 8.7396 3.0416 sensor resistance Offset −925.1580−898.3378 −178.2307 −199.0406 characteristic R @ 100° C. 58.3197 34.886831.8357 98.3183 (mohm)

As noted, sensor temperature control system 208 is explained with regardto temperature control of a medium sensor, namely sensor 112. Sensorcontrol system 208 preferably maintains sensor 112 at a constanttemperature by increasing or decreasing the power level fed to mediumsensor 112 in response to changes of sensor 112's measured resistance.With a fixed power level into sensor 112 and an increasing moisturelevel in the air flowing through sense module 110, the temperature willtend to drop and will be indicated by a decreasing R_(sensor) of mediumsensor 112. Conversely, as the moisture content in airflow passingthrough sensor module 110 decreases, the temperature of sensor 110 willtend to increase as indicated by an increasing R_(sensor). An exemplarycurve showing the relationship between sensor resistance and sensortemperature is shown in FIG. 6. In particular, FIG. 6 gives therelationship for sensor 112, that is preferably determined in atemperature controlled circulating oil bath when sensor 112 ismanufactured.

The stored values that characterize a temperature-resistancerelationship of medium sensor 112, for example, is characterized by twovalues, R@100 and dT/dR. The value R@100 gives medium sensor 112resistance in milliohms at 100° C. while the value dT/dR gives the slopeof the temperature vs. resistance line in terms of degrees C. permilliohm. Given the stored values of R@100, dT/dR and a desiredoperating temperature ST_(set) 228, processor 240 calculates a desiredresistance value indicative of the preferred ST_(set) which, in thepreferred embodiment, is 140° C. The power necessary to maintain adesired resistance value, and therefore temperature, is controlled bythe control system set forth in FIG. 4 and described as follows.

The output from controlled DC-DC converter 214, operating as a poweramplifier, preferably ranges between substantially 0 volts up tosubstantially 3 volts. A voltage across sensor 112 and current throughshunt 212, are measured and fed back to processor 240 via voltageamplifier 216 and current amplifier 218 respectively. The outputs of thevoltage amplifier 216 and current amplifier 218 are filtered by filters252 and 254 and fed to Analog to Digital (A/D) converters 220 and 222where they are converted to digital values. The digital values arepreferably supplied to a software routine represented by divider 224 andstored in processor 240 and wherein a substantially instantaneous valueof sensor 112's resistance is calculated. Likewise multiplier 236calculates a power value based on the fed back values of sensor 112'svoltage and current to give a value proportional to the power dissipatedin sensor 112. The output of multiplier 236 is sent to the control unit30 or 30A via a serial UART 244.

The output of the divider block 224, is fed to subtractor 226. Insubtractor 226 a present value of sensor 112's resistance from divider224 is compared to ST_(set) and the difference (resistance error signal230) is fed to PID (Proportional, Integral and Derivative) controller232. The error 230 is processed by the digital PID control loop whichcontinually calculates a new value to be fed to D/A converter 234, whereit is converted to an analog value and amplified by controlled DC to DCconverter 214. The output of controlled DC to DC converter 214 is thenfed to medium sensor 112 to raise or lower the sensor 112's power leveluntil the average resistance error 230 is zero and sensor 112'stemperature is at the preferred value of 140 degrees Centigrade.

PID controllers are often used for closed loop control and are known tothose skilled in the art. The basic principle of operation of the PIDloop is to output the sum of three terms, P, I, and D, which performdifferent specific functions. The Proportional, or P, term multipliesthe error by a constant and feeds it directly to the sum; if the wiretemperature falls below the set point, the P term immediately calls formore power. The Integral, or I, term integrates the error and applies itto the output sum. In this way small errors will accumulate over aperiod of time so that the average error is zero. The Derivative, or D,term applies a rate feedback term to the output sum to keep the systemfrom overshooting. We have discovered that, in the preferred embodimentof the temperature control systems for thin sensor 114, medium sensor112, scoop sensor 116 and compensation sensor 118 the D term is notrequired.

Preferably, D/A 234 has an adjustable minimum voltage setting that setsa minimum power level that will be delivered to medium sensor 112 toprevent divide by zero errors in a resistance calculation of divider 224of FIG. 4 for example. Preferably, this minimum power level is set lowenough to not substantially heat sensor 112 and introduce errors into apower calculation for the determination of wet or dry power and allowfor safe operation down to zero airspeed.

Deice Temperature Control System

A deice temperature control system 308 is located in temperaturecontroller 20 and maintains sense module shroud 120, strut 130 andfairing 134 at a substantially constant temperature to control heat lossfrom thin sensor 112, medium sensor 114, scoop sensor 116 andcompensation sensor 118 to the shroud. In the preferred embodimentshroud 120, strut 130 and fairing 134 are at ground potential and form areturn path for current through sensors 112, 114, 116 and compensationsensor 118 as well as for heater sections 264 and 266. The return pathconnection for sensors 112, 114, 116 is through the shroud at region 135of FIG. 2G and the connection for compensation sensor 118 is made bystud 138 as shown in FIG. 2G.

Deice temperature control system 308 includes deice strut heater 268,shroud heater 252 which is preferably in the form of a heater tapeplaced between inner shroud portion 126 and outer shroud portions 128.As illustrated in FIGS. 2G and 5, shroud heater 252 comprises fwd heaterportion 264 section and aft heater portion 266, both of which are fed byan electrical connection 262 in region 136 as shown in FIG. 2C. In thepreferred embodiment, the electrical connection forces the fwd portion264 of heater 252 (see 2G) to have a lower resistance than aft portion266 to provide more heat at the forward portion of the shroud which isfirst cooled by airstream 104. The lower resistance of the fwd section264 than the resistance of aft section 266 provides that the shroudheater 264 operates at a higher power level then aft heater 266.

Strut heater 268 and heater sections 264 and 266 are preferablyconnected in parallel and fed by pulse width modulator 314 having afixed frequency of approximately 895 hz. The temperature of the shroud120, fairing 134 and strut in the region of thermistor 140 is controlledby feeding back the resistance read at thermistor 140 and adjusting theoutput of pulse width modulator 314 to maintain thermistor 140 at aconstant temperature of preferably 50 degrees C.

One end of thermistor 140 is connected to fairing 135 which is at groundpotential and the other end is connected to A/D converter 324 viaamplifier 316 and RMS converter 352. A/D converter 324 preferablyincludes a current source output fed to thermistor 140 to convert theresistance of thermistor 140 into an analog voltage. The analog voltageis converted to its digital equivalent by A/D converter 324.

The output of A/D converter 324 is compared to a desired DeiceTemperature Set point (DTset) stored in processor 340 and a differencefrom subtractor 326 is fed to PID controller 332 and there to pulsewidth modulator 314. We have found that the “D” term of the PIDcontroller used in the Deice temperature control systems can be usefulfor maintaining the shroud 120, fairing 134 and strut 130 at a correcttemperature because of its greater thermal mass as compared individualsensors 112, 114, 116 and 118.

The power supplied to heaters 268, 264 and 266 is calculated by sensingheater voltage between sense point 262 and ground while current throughthe heaters is measured by shunt 312. The calculated power as well asthe resistance of thermistor is sent to control unit 30 or 30A and isused in the preferred embodiment as a backup indicator of total watercontent and a measure of the condition of the heaters 268, 264 and 266.

At low or zero airspeed, a minimum amount of deice power is required. Asthe airspeed increases, or ambient temperature decreases, the amount ofdeicing power will increase. Furthermore during encounters with liquidwater and or ice water, extra power will be delivered by the deicecontrol loop to compensate for the increased cooling of the impingingwater. Preferably up to 375 watts of heating power can be provided tothe shroud 120, fairing 134 and strut 134 heaters.

Calculation of Water Content

The cooling of compensation sensor 118, by virtue of its orientation, issubstantially independent of both the LWC and IWC of the air flowing inairstream 104 through sensor module 110. Thus, the power to hold thecompensation sensor 118 at a constant temperature is substantially onlya function of the airflow or dry power.

Preferably, the dry power value to subtract from the total powerprovided to each sensor 112, 114 and 116 is determined using the curvesof FIG. 7, which relate the dry power for compensation sensor 118 to adry power value for each of sensors 112, 114 and 116. For bestperformance, the curves of FIG. 7 are preferably generated in a windtunnel with a substantially water free airflow 104. In an alternateembodiment the curves would be generated from thermodynamicconsiderations.

Water content, whether originating as ice water or liquid water, ismeasured by measuring the power (wet power) to keep a sensor at a fixedtemperature while the water in contact with a sensor is evaporated.

When a droplet of liquid water impacts any of the sensors 112, 114 or116, two steps are necessary to evaporate the droplet. First the dropletmust be heated from its initial temperature to its evaporativetemperature, which at standard conditions requires 1 calorie/gram. Then,once at the evaporative temperature, an additional approximately 540cal/gram (corrected for the evaporative pressure when known), the latentheat of evaporation, will cause the droplet to evaporate.

Likewise it is known from theory that if the water to be evaporatedstarted in a glaciated state (i.e. as ice) as might be the case of thescoop sensor 116, an additional 80 cal/gram would be necessary toconvert the ice into water. We have found however that ignoring the 80cal per gram factor simplifies calculations without having a majorinfluence on results.

As one skilled in the art knows, the temperature at which a dropletbegins to rapidly evaporate, T_(evap), depends on ambient pressure andthat the latent heat of vaporization depends on the T_(evap) and methodsfor determining these corrections are therefore not further discussedfor purposes of brevity.

The next step in the calculation of liquid water content measured bysensor 112 and 114 and liquid water and or ice water content as measuredin scoop sensor 116, is to convert the power dissipated in each of thesensors, after correction for the dry power losses, into grams of watercurrently being evaporated and then, knowing the projected area of eachsensor and true airspeed, into grams per cubic meter. These conversionsare expressed in the following equation known to those skilled in theart:

${{LWC}\left( \frac{g}{m^{3}} \right)} = \frac{P_{{sense},{wet}}\mspace{14mu}{({watts}) \cdot 2.389} \times 10^{5}}{\begin{matrix}{\left\lbrack {{L_{evap}\frac{cal}{g}} + {1.0\;\frac{cal}{g \cdot {{^\circ}C}}\left( {T_{evap} - T_{ambient}} \right)}} \right\rbrack \cdot} \\{{TAS}\;{\frac{m}{s} \cdot L_{sense}}\mspace{14mu}{{mm} \cdot W_{sense}}\mspace{14mu}{mm}}\end{matrix}}$Where:P_(sense,wet)=Wet Power measured by the sense element wattsL_(evap)=Latent heat of evaporation cal/gm as corrected for evaporativetemperatureT_(evap)=Evaporative temperature in ° C. as corrected for AmbientpressureT_(ambient)=Static Ambient temperature in ° C.L_(sense)=Length of the sense element in mmW_(sense)=Width of the sense element in mmTAS=True airspeed in m/s

The equation above gives the water evaporated by each of the mediumsensor 112, thin sensor 114 and scoop sensor 116.

We have confirmed in wind tunnel tests that the design and placement ofscoop sensor 116 tends to capture and evaporate liquid water and icewater without affecting the response of the medium or thin sensors 112and 114. The result is that heat lost from scoop 116, after subtractionof dry power, is a measure of the TWC (the sum of LWC and IWC) in acloud. Likewise, and as discussed, the powers to keep medium sensor 112and thin sensor 114 at a constant temperature (after subtraction of thedry power) are measures of the LWC water in airstream 104 through thesensor module 110.

Moreover it is known that larger diameter drop sizes tend not to stickto the thin sensor 114 long enough to evaporate while they do tend tostick to the medium sensor 112. Likewise smaller drops tend to passaround the medium sensor but stick to the thin sensor. This behaviorresults in different measurements of LWC using medium sensor 112 andthin sensor 114 and this difference is useful in estimating MVD.

The calculations above are performed in Control unit 30 and 30 a usingpower data transmitted from temperature controller 208 via UART 244 asshown in FIG. 4.

Estimation of MVD

The difference in response of the medium sensor and thin sensor todroplet size, when expressed as a ratio of wet power of medium sensor112 to wet power of thin sensor 114, and calibrated in a wind tunnel,has been found to be a repeatable discriminator of the MVD of cloudliquid water. Calibration is accomplish by in a wind tunnel using acalibrated source of known MVD, logging the ratio of wet powers of themedium and thin sensor as a function of MVD and storing the results inEPROM 424 located in Control Unit 30 and 30 a and shown in FIGS. 10 and11. FIG. 8 shows calibration results from wind tunnel measurements forthe preferred embodiment at true airspeeds speeds of up to 100 metersper second and MVD's up to 100 microns.

Detection and Measurement of Ice Water

Since the heat loss from the preferred medium sensor 112 and preferredthin sensor 114 do not substantially change in the presence of the icewater but the scoop sensor 116 does show increased heat loss in thepresence of liquid water or ice water, the invention uses these responsecharacteristics to discriminate between the presence of the differentwater phases as described below.

If either the medium sensor 112 or thin sensor 114 show LWC due tomoisture AND the TWC from the scoop sensor 116 is greater than thelarger of the LWC from the medium sensor 1120R thin sensor 114 then thelarger of the LWC calculations from the medium or thin sensor gives ameasure of the LWC and the TWC contains both liquid water and ice water,i.e. the cloud is of mixed phase. Subtraction of LWC from TWC gives ameasure of the IWC.

If the TWC given by the scoop sensor 116 is substantially equal to thelarger of the value from the medium or thin sensor, then the cloud wateris substantially all water, the TWC is substantially equal to the LWC.

Moreover, if the scoop indicates heat loss but neither the medium orthin sensor indicates meaningful heat loss due to moisture, then thewater in the cloud is substantially all ice water.

An alternate embodiment of the invention provides corrected measurementand detection accuracy of LWC and IWC as follows. The invention sensehead 10 is placed in a wind tunnel or other calibrated environment. ALWC ratio is formed by measurement of LWC as measured by thin sensor114, to a LWC measurement as measured by a wind tunnel reference deviceand the ratio is recorded as a function of MVD as given by an MVDreference device. In a preferred embodiment, a best fit curve in theform of an exponential is formed using the ratio and MVD values within aspread sheet program. Data representing the best fit curve is preferablestored in the form of a lookup table in the EPROM 424 of control unit 30or 30 a. Test data and a best fit curve in the form of an exponentialfor thin sensor 114 are provided in FIG. 9.

This same process is repeated for medium sensor 112 and scoop sensor116.

In a flight environment, the invention calculates initial LWC valuesfrom medium sensor 112 and thin sensor 114 based on wet powermeasurements as previously described, and then selects the higher of thetwo values storing both the higher value and an indicator of which ofthe sensors experienced the higher value.

As an example, for a case where thin sensor 114 gives a higher value ofLWC, the invention uses a substantially simultaneously determined valueof MVD (by the ratio of wet powers of medium sensor 112 to the wet powerof thin sensor 114 as a function of MVD curve) to enter a look up tablebased on FIG. 9 to yield an LWC correction factor. The inverse of thisfactor is then multiplied by the higher LWC to yield a corrected LWC.

Moreover subtraction of a corrected LWC from TWC gives an improved valueof IWC.

Power Fluctuations Due to Approaching Cloud Water or Ice

An alternate embodiment of the invention combines measurement of LWC andIWC using the wet power measurements with information gained fromfluctuations of total power for each sensor that are experienced beforesignificant measurements of wet power as described below.

It was observed during testing in both wind tunnels and on-boardaircraft that, as the amount of cloud water begins to rise, but prior tothere being a significant level of average wet power delivered to eitherof the medium 112, thin 114 or scoop 116 sensors to establish that thesensor head 10 is actually in a cloud, fluctuations in total power (wetplus dry) to either or all of the medium 112, thin 114 and scoop 116sensors can increase noticeably.

The rapid response of the sensors 112, 114 and 116 in cooperation withthe temperature control systems represented by temperature controlsystem 208 gives changing sensor power values (wet plus dry) in responseto random fluctuations in the amount of water intercepted by the sensor.This response, which is estimated to be less 0.01 seconds, is usefuleven though the cloud is, in general, quite uniform (over a largedistance) while having little appreciable water. Said another way, whilethe average amount of water over a large volume of the cloud may besmall, the sensors none the less respond to small pockets of water inthe cloud.

Furthermore as the amount of cloud water increases, the powerfluctuations diminish and the calculated LWC and IWC measurementindicate the presence of water in either liquid or ice form.

Calculation of Power Fluctuations

In operation, a temperature controller 208, in order to maintain asensor at a preferred temperature of 140 degrees C., adjusts the powerlevel to the sensor multiple times per second, the preferred rate beingtwenty to forty times per second.

The presence of pockets of liquid water or ice water is determined (foreach sensor) as follows:

-   -   1. All the power level samples for a given period of time,        typically one second, are averaged together giving an average        power level for the particular time period.    -   2. The average value calculated in Step 1 is subtracted from        each individual power sample taken in the time period, giving        the departure from average for each power sample.    -   3. The absolute value of each sample's departure is then        calculated. The result is the magnitude of each samples        fluctuation around the average    -   4. An average of all the individual, absolute values is then        computed.    -   5. If the magnitude of this averaged value exceeds the specified        threshold value associated with each sensor given below, then        the associated sensor is exposed to liquid water and or ice        water as indicated.

The steps 1 through 5 are performed for the power values for each ofsensors 112, 114 and 116 and the resulting values are used in thedisplay logic given in a paragraph below. It is noted that the magnitudeof the power fluctuations developed above are termed: “medium sensorpower fluctuations”, “thin sensor power fluctuations” and “scoop sensorpower fluctuations”.

In an alternate embodiment, a Fourier analysis is done on the stream ofpower level samples to provide an RMS value of the resulting spectrum(with dc term removed) and therefore an indication of the level of powerfluctuations.

These calculations of sensor power fluctuations are performed in controlunits 30 or 30A.

Data Reduction and Display

Data reduction processes and display functions are preferably performedin processor 418 of Control Unit 30 as shown in FIG. 10 and based onpower calculations performed in a temperature controller for each sensorrepresentatively displayed by temperature controller 208 shown in FIG.4. Processor 418 in control unit 30 receives multiplex serial power data456 from each temperature controller via UART 414 in control unit 30 aswell as ambient data 454 from a host aircraft (or wind tunnel facilityif appropriate) via an ARINC 429 link. Common data stream 456 alsoincludes power supplied to deice heaters 262, 264 and 266 via UART 414of FIG. 5. Common stream 456 is formed in a multiplexer (not shown) thatcombines the outputs of all UARTS represented by UART 244 and UART 344of FIG. 5.

Processor 418 accesses RAM 420, EPROM 424, which stores calibrationdata, and ROM 422 having program data stored therein. Processor 418 alsoprovides a signal to cockpit warning indicator light 426 when an unsafeor potentially unsafe condition occurs in flight.

Threshold driven indicators present in control unit 30 and control unit30 a are controlled as follows. Note that like reference numbers referto similar items and functionality for both control units 30 and 30A.

IWC Indicator 402 illuminates if the result of performing the followinglogic yields a logical “TRUE” signal:

-   -   IF ((Calculated IWC exceeds a specified threshold #1) OR ((Scoop        sensor power fluctuations exceed a specified threshold #2) AND        (medium sensor power fluctuations do not exceed a specified        threshold #3) AND (thin sensor power fluctuations do not exceed        specified threshold #4)))        SLD (Super-cooled Large Droplets) Indicator 404 illuminates if        the result of performing the following logic yields a logical        “TRUE” signal:    -   IF ((Calculated MVD exceeds a specified threshold #5) AND        ((Calculated LWC from the medium sensor exceeds specified        threshold #6) OR (Calculated LWC from the thin sensor exceeds a        specified threshold #7)) AND (Air temperature is below specified        threshold #8))        LWC Indicator 406 illuminates if the result of performing the        following logic yields a logical “TRUE” signal:    -   IF ((Calculated LWC from the medium sensor exceeds specified        threshold #9) OR (Calculated LWC from thin sensor exceeds        specified threshold #10))        IN Cloud indicator 408 illuminates if the result of performing        the following logic yields a logical “TRUE” signal:

IF ((Measured LWC from the thin sensor exceeds specified threshold #11)OR (Measured LWC from the medium sensor exceeds specified threshold #12)OR (Measured TWC from the scoop sensor exceeds specified threshold #13)OR (Thin sensor power fluctuations exceeds specified threshold #14) OR(Medium sensor power fluctuations exceeds specified threshold #15) OR(Scoop sensor power fluctuations exceeds specified threshold #16))

Potential Icing Indicator 410 illuminates if the result of performingthe following logic yields a logical “TRUE” signal:

-   -   IF ((In Cloud indicator 408 is illuminated) AND (Air temperature        is below specified threshold #17))

Where sensor power fluctuations are fluctuations of total power to asensor, that is, wet power plus dry power and:

threshold #1 is preferably 0.020 gm/m3; threshold #2 is preferably 0.080watts; threshold #3 is preferably 0.040 watts; threshold #4 ispreferably 0.010 watts; threshold #5 is preferably 50.0 um; threshold #6is preferably 0.020 gm/m3; threshold #7 is preferably 0.020 gm/m3;threshold #8 is preferably +2.0 deg Centigrade; threshold #9 ispreferably 0.020 gm/m3; threshold #10 is preferably 0.020 gm/m3;threshold #11 is preferably 0.020 gm/m3; threshold #12 is preferably0.020 gm/m3; threshold #13 is preferably 0.020 gm/m3; threshold #14(same as threshold #4) is preferably 0.010 watts; threshold #15 (same asthreshold #3) is preferably 0.040 watts; threshold #16 (same asthreshold #2) is preferably 0.080 watts; and threshold #17 is preferably+2.0 deg Centigrade.

As one skilled in the art would appreciate, alternate embodiments ormodifications to the invention as well as desired false alarm ratescould affect a choice of thresholds.

Control Unit 30 includes an LCD display 444 which, in addition todisplaying the indicators described above, also provide measurement datain analog form as described as follows. Bar graph 428 indicates MVD overthe preferred range of 0-100 microns or alternately in excess of 250microns. Bar graphs 430, 432, 434 and 436 indicates IWC, TWC (from Scoopsensor 116) LWC from medium sensor 112 and thin sensor 114 over apreferred range of about 0 to about 3 g/m³ and alternative over therange of about 0 to about 5 g/m³. Control unit 30 also includes timehistory 442 to shown present and past values of TWC, IWC and the largerof LWC measured by the medium sensor or thin sensors.

Control Unit 30 is preferably used in a research environment such as aresearch aircraft or wind tunnel facility. In a commuter, commercial,private or military aircraft an alternate embodiment as shown in FIG. 11at 30A is preferred. The primary difference between the embodiments 30and 30A is that in control unit 30 only the threshold indicators 402,404, 406, 408 and 410 are used. In addition, processor 418 of controlunit 30 and shown in FIG. 10 preferrably has increased processingpower/speed when compared to processor 416 of control unit 30A toaccommodate calculations needed to control the display of measuredvalues.

It can thus be seen that the present invention provides an improvedsystem and method for the calculation of cloud parameters previouslyonly possible by using multiple instruments on research aircraft. Inparticular, the present invention provides an improved system and methodfor providing a statistical measure of the size of liquid waterdroplets, for detection and/or measurement of the presence of a cloudand/or LWC, IWC and SLD parameters; for overcoming difficulties ofduplicating cloud conditions, and MVD in particular, in wind tunneltesting; as well as an improved system that is lightweight and suitablefor use on non-research aircraft.

For example, it can thus be seen that in one preferred embodiment, thepresent invention is directed to a system for determining a measure ofcloud water droplet size based on sensor cooling by cloud water andstored sensor power ratio and droplet size data, the system comprising afirst and second temperature controlled co-located self heated sensors,the cooling of the sensors being substantially independent of ice water;a self heated compensation sensor oriented to be substantiallyindependent of cloud water cooling; a variable power source coupled toeach first and second sensor for keeping each sensor at a temperatureand each having a measurable output power; a variable power sourcecoupled to the compensation sensor for keeping the sensor at atemperature and having a measurable output power; and a control unit forcorrecting the measured power to each of the first and second sensors toreflect only heat loss due to cloud water by subtracting from themeasured power an adjusted compensation sensor measured value; andforming a ratio of corrected power of each of the first and secondsensors to identify a liquid water droplet size based on the storedpower ratio and droplet size data.

In another embodiment, the present invention is directed to a system formeasuring water droplet size in a cloud, wherein the system comprises asensor head comprising a first heated sensor having a resistancecharacteristic that is temperature dependent, wherein power loss fromthe sensor is at least essentially not affected by the presence of icewater in an airflow thereacross; a second heated sensor having a secondresistance characteristic that is temperature dependent, wherein powerloss from the second sensor is at least essentially not affected by thepresence of ice water in the airflow thereacross; a heated compensationsensor being positioned to be responsive to the airflow thereacross andhaving a power loss at least essentially not affected by the presence ofliquid water or ice water in the airflow thereacross; a respectivetemperature controller associated with each of the first sensor, secondsensor and compensation sensor, each of which for maintaining thetemperature of the respective sensors at respective temperature valuesusing its temperature dependent resistance characteristic, wherein, foreach of the first and second sensors, the respective controllermaintains the sensor at the temperature value by adjusting a power levelfed to the sensor; calculates the total power required to maintain thesensor at the temperature value when the sensor is exposed to an airflowcomprising at least one of liquid water and ice water; and a controlunit for receiving power levels from each of the first sensor and secondsensor; subtracting, from each total power calculation, a dry powercomponent determined from the power required to maintain thecompensation sensor at its temperature value in the presence of theairflow comprising at least liquid water, thereby arriving at a wetpower value attributable to the incremental power associated withmaintaining the respective first and second sensors at the respectivetemperature values in the presence of at least liquid water in theairflow; and for forming a ratio of the wet power value of the firstsensor to the wet power value of the second sensor to obtain anestimated value of the measure of water droplet size in the cloud from apredetermined calibrated relationship of wet power ratio to waterdroplet sizes.

In yet another embodiment, the present invention is directed to a systemfor determining liquid water content in a cloud, wherein the systemcomprises a sensor head comprising a first heated sensor having a firstresistance characteristic that is temperature dependent, wherein thefirst sensor is at least essentially not affected by the presence of icewater in an airflow thereacross; a second heated sensor having a secondresistance characteristic that is temperature dependent, wherein thesecond sensor is at least essentially not affected by the presence ofice water in the airflow thereacross; a heated compensation sensor beingpositioned to be responsive to the airflow thereacross and at leastessentially not affected by the presence of liquid water or ice water inthe airflow thereacross; a respective temperature controller associatedwith each of the first sensor, second sensor and compensation sensor,each of which for maintaining the temperature of the respective sensorsat respective temperature values using its temperature dependentresistance characteristic, wherein, for each of the first and secondsensors the respective controller maintains the sensor at thetemperature value by adjusting a power level fed to the sensor;calculates the total power required to maintain the sensor at thetemperature value when the sensor is exposed to an airflow comprising atleast liquid water; and a control unit for receiving power levels fromeach of the first sensor and second sensor; subtracting, from each totalpower calculation, a dry power component determined from the powerrequired to maintain the compensation sensor at its temperature value inthe presence of the airflow thereby arriving at a wet power valueattributable to the incremental power associated with maintaining therespective first and second sensors at the respective temperature valuesin the presence of at least liquid water in the airflow; and calculatinga liquid water content for the first sensor and sensor wet power valuesand selecting the higher LWC as the desired liquid water content.

In still another embodiment of the present invention, a system isprovided for determining liquid water content in a cloud, the liquidwater having a determined droplet size, wherein the system comprises asensor head comprising a first heated sensor having a first resistancecharacteristic that is temperature dependent, wherein the first sensoris at least essentially not affected by the presence of ice water in anairflow thereacross; a second heated sensor having a second resistancecharacteristic that is temperature dependent, wherein the second sensoris at least essentially not affected by the presence of ice water in theairflow thereacross; a heated compensation sensor being positioned to beresponsive to the airflow thereacross and at least essentially notaffected by the presence of liquid water or ice water in the airflowthereacross; a respective temperature controller associated with each ofthe first sensor, second sensor and compensation sensors, each of whichfor maintaining the temperature of the respective sensors at respectivetemperature values using its temperature dependent resistancecharacteristic, wherein, for each of the first and second sensors therespective controller maintains the sensor at the temperature value byadjusting a power level fed to the sensor; calculates the total powerrequired to maintain the sensor at the temperature value when the sensoris exposed to an airflow comprising at least liquid water; and a controlunit for receiving power levels from each of the first sensor and secondsensor and having a stored droplet size verses liquid water contentcorrection curve for each first and second sensor; subtracting, fromeach total power calculation, a dry power component determined from thepower required to maintain the compensation sensor at its temperaturevalue in the presence of the airflow thereby arriving at a wet powervalue attributable to the incremental power associated with maintainingthe respective first and second sensors at the respective temperaturevalues in the presence of liquid water in the airflow; and calculating aliquid water content for the first sensor and second sensor wet powervalues, selects the higher of the calculated liquid water contentvalues, and corrects the higher of the two values using a droplet sizeverses liquid water content correction curve.

In yet another embodiment, a system is provided for determining thepresence of ice water in an airflow, wherein the system comprises asensor head comprising a first heated sensor having a first resistancecharacteristic that is temperature dependent, wherein heat the fromfirst sensor is at least essentially not affected by the presence of icewater in the airflow thereacross; a second heated sensor having a secondresistance characteristic that is temperature dependent, wherein heatfrom the second sensor is at least essentially not affected by thepresence of ice water in the airflow thereacross; a third heated sensorhaving a third resistance characteristic that is temperature dependent,wherein heat loss from the third sensor is affected by a presence of icewater in the airflow thereacross; a heated compensation sensor beingpositioned to be responsive to the airflow thereacross and having a heatloss least essentially not affected by the presence of liquid water orice water in the airflow thereacross; a respective temperaturecontroller associated with each of the first sensor, second sensor,third sensor and compensation sensors, each of which for maintaining thetemperature of the respective sensors at respective temperature valuesusing its temperature dependent resistance characteristic, wherein, foreach of the first, second and third sensors, the respective controllermaintains the sensor at the temperature value by adjusting a power levelfed to the sensor; calculates the total power required to maintain thesensor at the temperature value when the sensor is exposed to an airflowcomprising at least ice water; and a control unit for receiving powerlevels from each of the first sensor and second sensor; subtracting,from each total power calculation, a dry power component determined fromthe power required to maintain the compensation sensor at itstemperature value in the presence of the airflow comprising at least icewater, thereby arriving at a wet power value attributable to theincremental power associated with maintaining the respective first,second and third sensors at the respective temperature values in thepresence of at least ice water in the airflow; and calculating a watercontent measurement for the first sensor, second sensor and third sensorwet power values, selecting a higher water content value from the firstand second sensor and subtracting the higher value from the watercontent value of the third sensor to indicate a presence of ice water.

In still another embodiment, a system is provided for warning of cloudwater, the system comprising a first temperature controlled self heatedsensor, a variable power source for the first sensor having a measurableoutput power responsive to changes in cloud water; a processor foraveraging the measured output power of the power source and subtractinga substantially instantaneous measure of the output power of the powersource to determine fluctuations around the average; and comparing thefluctuations to a threshold indicative of the presence of cloud water.

In still further embodiments, the aforementioned constructions could bemodified by those skilled in the art while remaining within the scope ofthe present invention. Thus, the invention generally covers means forcarrying out the aforementioned functionality. For example, a singlecontroller or control unit could carry out the aforementionedfunctionality if so desired, and therefore, the invention is not limitedby the specific construction set forth above.

As should now be seen, the present invention is also directed to aplurality of methodologies. For example, in accordance with a firstembodiment, a method for determining a measure of cloud water dropletsize based on sensor cooling by cloud water and stored sensor powerratio and droplet size data, utilizing a system comprising a first andsecond temperature controlled co-located self heated sensors, thecooling of the sensors being substantially independent of ice water, aself heated compensation sensor oriented to be substantially independentof cloud water cooling; a variable power source coupled to each firstand second sensor for keeping each sensor at a temperature and eachhaving a measurable output power; and a variable power source coupled tothe compensation sensor for keeping the sensor at a temperature andhaving a measurable output power, wherein the method comprises the stepsof correcting the measured power to each of the first and second sensorsto reflect only heat loss due to cloud water by subtracting from themeasured power an adjusted compensation sensor measured value; andforming a ratio of corrected power of each of the first and secondsensors to identify a liquid water droplet size based on the storedpower ratio and droplet size data.

In yet another embodiment, the present invention is directed to a methodof measuring water droplet size in an airflow, utilizing a systemcomprising a sensor head comprising a first heated sensor having aresistance characteristic that is temperature dependent, wherein powerloss from the sensor is at least essentially not affected by thepresence of ice water in the airflow thereacross; a second heated sensorhaving a second resistance characteristic that is temperature dependent,wherein power loss from the second sensor is at least essentially notaffected by the presence of ice water in the airflow thereacross; aheated compensation sensor being positioned to be responsive to theairflow thereacross and having a power loss at least essentially notaffected by the presence of liquid water or ice water in the airflowthereacross, wherein the method comprises the steps of maintaining thetemperature of the respective sensors at respective temperature valuesusing its temperature dependent resistance characteristic; and wherein,for each of the first and second sensors, the method comprises the stepsof maintaining the sensor at the temperature value by adjusting a powerlevel fed to the sensor and calculating the total power required tomaintain the sensor at the temperature value when the sensor is exposedto an airflow comprising at least one of liquid water and ice water;wherein the method further comprises the steps of receiving power levelsfrom each of the first sensor and second sensor; subtracting, from eachtotal power calculation, a dry power component determined from the powerrequired to maintain the compensation sensor at its temperature value inthe presence of the airflow comprising at least liquid water, therebyarriving at a wet power value attributable to the incremental powerassociated with maintaining the respective first and second sensors atthe respective temperature values in the presence of at least liquidwater in the airflow; and forming a ratio of the wet power value of thefirst sensor to the wet power value of the second sensor to obtain anestimated value of the measure of water droplet size in the cloud from apredetermined calibrated relationship of wet power ratio to waterdroplet sizes.

In still yet another embodiment, a method is provided for determiningliquid water content in an airflow, utilizing a system comprising asensor head comprising a first heated sensor having a first resistancecharacteristic that is temperature dependent, wherein the first sensoris at least essentially not affected by the presence of ice water in anairflow thereacross; a second heated sensor having a second resistancecharacteristic that is temperature dependent, wherein the second sensoris at least essentially not affected by the presence of ice water in theairflow thereacross; a heated compensation sensor being positioned to beresponsive to the airflow thereacross and at least essentially notaffected by the presence of liquid water or ice water in the airflowthereacross; wherein the method comprises the steps of maintaining thetemperature of the respective sensors at respective temperature valuesusing its temperature dependent resistance characteristic, maintainingthe sensors at the temperature value by adjusting a power level fed tothe sensor; calculating the total power required to maintain the sensorat the temperature value when the sensor is exposed to an airflowcomprising at least liquid water; receiving power levels from each ofthe first sensor and second sensor; subtracting, from each total powercalculation, a dry power component determined from the power required tomaintain the compensation sensor at its temperature value in thepresence of the airflow thereby arriving at a wet power valueattributable to the incremental power associated with maintaining therespective first and second sensors at the respective temperature valuesin the presence of at least liquid water in the airflow; and calculatinga liquid water content for the first sensor and sensor wet power valuesand selecting the higher LWC as the desired liquid water content.

Still in another embodiment, a method is provided for determining liquidwater content in an airflow, the liquid water having a determineddroplet size, utilizing a system comprising a sensor head comprising afirst heated sensor having a first resistance characteristic that istemperature dependent, wherein the first sensor is at least essentiallynot affected by the presence of ice water in an airflow thereacross; asecond heated sensor having a second resistance characteristic that istemperature dependent, wherein the second sensor is at least essentiallynot affected by the presence of ice water in the airflow thereacross; aheated compensation sensor being positioned to be responsive to theairflow thereacross and at least essentially not affected by thepresence of liquid water or ice water in the airflow thereacross;wherein the method comprises the steps of maintaining the temperature ofthe respective sensors at respective temperature values using itstemperature dependent resistance characteristic and, for each of thefirst and second sensors, maintaining maintains the sensor at thetemperature value by adjusting a power level fed to the sensor andcalculating the total power required to maintain the sensor at thetemperature value when the sensor is exposed to an airflow comprising atleast liquid water; wherein the method further comprises the steps ofreceiving power levels from each of the first sensor and second sensorand having a stored droplet size verses liquid water content correctioncurve for each first and second sensor; subtracting, from each totalpower calculation, a dry power component determined from the powerrequired to maintain the compensation sensor at its temperature value inthe presence of the airflow thereby arriving at a wet power valueattributable to the incremental power associated with maintaining therespective first and second sensors at the respective temperature valuesin the presence of liquid water in the airflow; and calculating a liquidwater content for the first sensor and second sensor wet power values,selects the higher of the calculated liquid water content values, andcorrects the higher of the two values using a droplet size verses liquidwater content correction curve.

In yet another embodiment, a method is provided for determining thepresence of ice water in an airflow, utilizing a system comprising asensor head comprising a first heated sensor having a first resistancecharacteristic that is temperature dependent, wherein heat the fromfirst sensor is at least essentially not affected by the presence of icewater in the airflow thereacross; a second heated sensor having a secondresistance characteristic that is temperature dependent, wherein heatfrom the second sensor is at least essentially not affected by thepresence of ice water in the airflow thereacross; a third heated sensorhaving a third resistance characteristic that is temperature dependent,wherein heat loss from the third sensor is affected by a presence of icewater in the airflow thereacross; a heated compensation sensor beingpositioned to be responsive to the airflow thereacross and having a heatloss least essentially not affected by the presence of liquid water orice water in the airflow thereacross; wherein the method comprises thesteps of maintaining the temperature of the respective sensors atrespective temperature values using its temperature dependent resistanceand wherein, for each of the first, second and third sensors,maintaining the sensor at the temperature value by adjusting a powerlevel fed to the sensor and calculating the total power required tomaintain the sensor at the temperature value when the sensor is exposedto an airflow comprising at least ice water; wherein the method furthercomprises the steps of receiving power levels from each of the firstsensor and second sensor; subtracting, from each total powercalculation, a dry power component determined from the power required tomaintain the compensation sensor at its temperature value in thepresence of the airflow comprising at least ice water, thereby arrivingat a wet power value attributable to the incremental power associatedwith maintaining the respective first, second and third sensors at therespective temperature values in the presence of at least ice water inthe airflow; and calculating a water content measurement for the firstsensor, second sensor and third sensor wet power values, selecting ahigher water content value from the first and second sensor andsubtracting the higher value from the water content value of the thirdsensor to indicate a presence of ice water.

In still another preferred embodiment of the present invention, a methodis provided for warning of cloud water using a first temperaturecontrolled self heated sensor and a variable power source for the firstsensor having a measurable output power responsive to changes in cloudwater, wherein the method comprises the steps of averaging the measuredoutput power of the power source; subtracting a substantiallyinstantaneous measure of the output power of the power source todetermine fluctuations around the average; and comparing thefluctuations to a threshold indicative of the presence of cloud water.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are efficiently attained and,since certain changes may be made in the above constructions withoutdeparting from the spirit and scope of the invention, it is intendedthat all matter contained in the above description or shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

It should also be understood that the following claims are intended tocover all of the generic and specific features of the inventiondescribed herein and all statements of the scope of the invention thatas a matter of language might fall therebetween.

1. A system for determining a measure of liquid water droplet size basedon sensor cooling by cloud water and stored sensor power ratio anddroplet size data, said system comprising: first and second temperaturecontrolled co-located self heated sensors, the cooling of said sensorsbeing substantially independent of ice water; a self heated compensationsensor oriented to be substantially independent of cloud water cooling;a variable power source coupled to each of the first and second sensorsfor keeping each sensor at a predetermined temperature and each variablepower source having an output power; a variable power source coupled tothe compensation sensor for keeping the sensor at a predeterminedtemperature and having an output power; and a control unit for:correcting a measured power to each of the first and second sensors toreflect only heat loss due to cloud water by subtracting from themeasured power to each of the first and second sensors adjustedcompensation sensor measured values; and forming a ratio of correctedpower of each of the first and second sensors to identify a liquid waterdroplet size based on the stored sensor power ratio and droplet sizedata.
 2. A system for measuring water droplet size in a cloud, whereinthe system comprises: a sensor head comprising: a first heated sensorhaving a resistance characteristic that is temperature dependent,wherein power loss from the sensor is at least essentially not affectedby the presence of ice water in an airflow thereacross; a second heatedsensor having a second resistance characteristic that is temperaturedependent, wherein power loss from the second sensor is at leastessentially not affected by the presence of ice water in the airflowthereacross; a heated compensation sensor being positioned to beresponsive to the airflow thereacross and having a power loss at leastessentially not affected by the presence of liquid water or ice water inthe airflow thereacross; a respective temperature controller associatedwith each of the first sensor, second sensor and compensation sensor,each of which for maintaining the temperature of the respective sensorsat respective temperature values using each sensor's temperaturedependent resistance characteristic, wherein, for each of the first andsecond sensors, the respective controller: maintains the sensor at thetemperature value by adjusting a power level fed to the sensor;calculates the total power required to maintain the sensor at thetemperature value when said sensor is exposed to an airflow comprisingat least one of liquid water and ice water; and a control unit for:receiving power levels from each of the first sensor and second sensor;subtracting, from each total power calculation, a dry power componentdetermined from the power required to maintain the compensation sensor'stemperature value in the presence of the airflow comprising at leastliquid water, thereby arriving at a wet power value attributable to anincremental power associated with maintaining the respective first andsecond sensors at the respective temperature values in the presence ofat least liquid water in the airflow; and forming a ratio of the wetpower value of the first sensor to the wet power value of the secondsensor to obtain an estimated value of the measure of water droplet sizein the cloud from a predetermined calibrated relationship of wet powerratios to water droplet sizes.
 3. A system for determining liquid watercontent in a cloud, wherein the system comprises: a sensor headcomprising: a first heated sensor having a first resistancecharacteristic that is temperature dependent, wherein the first sensoris at least essentially not affected by the presence of ice water in anairflow thereacross; a second heated sensor having a second resistancecharacteristic that is temperature dependent, wherein the second sensoris at least essentially not affected by the presence of ice water in theairflow thereacross; a heated compensation sensor being positioned to beresponsive to the airflow thereacross and at least essentially notaffected by the presence of liquid water or ice water in the airflowthereacross; a respective temperature controller associated with each ofthe first sensor, second sensor and compensation sensor, each of whichfor maintaining the temperature of the respective sensors at respectivetemperature values using the respective sensors' temperature dependentresistance characteristic, wherein, for each of the first and secondsensors the respective controller: maintains the sensor at thetemperature value by adjusting a power level fed to the sensor;calculates the total power required to maintain the sensor at thetemperature value when said sensor is exposed to an airflow comprisingat least liquid water; and a control unit for: receiving power levelsfrom each of the first sensor and second sensor; subtracting, from eachtotal power calculation, a dry power component determined from the powerrequired to maintain the compensation sensor's temperature value in thepresence of the airflow thereby arriving at a wet power valueattributable to an incremental power associated with maintaining therespective first and second sensors at the respective temperature valuesin the presence of at least liquid water in the airflow; and calculatinga liquid water content for the first and second sensor wet power valuesand selecting a higher LWC as the determined liquid water content.
 4. Asystem for determining liquid water content in a cloud, said liquidwater having a determined droplet size, wherein the system comprises: asensor head comprising: a first heated sensor having a first resistancecharacteristic that is temperature dependent, wherein the first sensoris at least essentially not affected by the presence of ice water in anairflow thereacross; a second heated sensor having a second resistancecharacteristic that is temperature dependent, wherein the second sensoris at least essentially not affected by the presence of ice water in theairflow thereacross; a heated compensation sensor being positioned to beresponsive to the airflow thereacross and at least essentially notaffected by the presence of liquid water or ice water in the airflowthereacross; a respective temperature controller associated with each ofthe first sensor, second sensor and compensation sensors, each of whichfor maintaining the temperature of the respective sensors at respectivetemperature values using the respective sensors' temperature dependentresistance characteristic, wherein, for each of the first and secondsensors the respective controller: maintains the sensor at thetemperature value by adjusting a power level fed to the sensor;calculates the total power required to maintain the sensor at thetemperature value when said sensor is exposed to an airflow comprisingat least liquid water; and a control unit for: receiving power levelsfrom each of the first sensor and second sensor and having a storeddroplet size verses liquid water content correction curve for each firstand second sensor; subtracting, from each total power calculation, a drypower component determined from the power required to maintain thecompensation sensor at the compensation sensor's temperature value inthe presence of the airflow thereby arriving at a wet power valueattributable to an incremental power associated with maintaining therespective first and second sensors at the respective temperature valuesin the presence of liquid water in the airflow; and calculating a liquidwater content value for the first sensor and second sensor wet powervalues, selecting the higher of the calculated liquid water contentvalues, and correcting the higher value using a droplet size versesliquid water content correction curve.
 5. A system for determining thepresence of ice water in an airflow, wherein the system comprises: asensor head comprising: a first heated sensor having a first resistancecharacteristic that is temperature dependent, wherein heat from thefirst sensor is at least essentially not affected by the presence of icewater in the airflow thereacross; a second heated sensor having a secondresistance characteristic that is temperature dependent, wherein heatfrom the second sensor is at least essentially not affected by thepresence of ice water in the airflow thereacross; a third heated sensorhaving a third resistance characteristic that is temperature dependent,wherein heat loss from the third sensor is affected by a presence of icewater in the airflow thereacross; a heated compensation sensor beingpositioned to be responsive to the airflow thereacross and having a heatloss least essentially not affected by the presence of liquid water orice water in the airflow thereacross; a respective temperaturecontroller associated with each of the first sensor, second sensor,third sensor and compensation sensor, each of which for maintaining thetemperature of the respective sensors at respective temperature valuesusing the respective sensors' temperature dependent resistancecharacteristic, wherein, for each of the first, second and thirdsensors, the respective controller: maintains the sensor at thetemperature value by adjusting a power level fed to the sensor;calculates the total power required to maintain the sensor at thetemperature value when said sensor is exposed to an airflow comprisingat least ice water; and a control unit for: receiving power levels fromeach of the first sensor and second sensor; subtracting, from each totalpower calculation, a dry power component determined from the powerrequired to maintain the compensation sensor at the compensationsensor's temperature value in the presence of the airflow comprising atleast ice water, thereby arriving at a wet power value attributable toan incremental power associated with maintaining the respective first,second and third sensors at the respective temperature values in thepresence of at least ice water in the airflow; and calculating a watercontent measurement for the first sensor, second sensor and third sensorwet power values, selecting a higher water content value from the firstand second sensor and subtracting the higher value from the watercontent value of the third sensor to indicate a presence of ice water.6. The system as claimed in claim 5, wherein if the difference betweenthe higher water content value and the water content value of the thirdsensor is greater than a threshold, then ice water is present.
 7. Thesystem as claimed in claim 5, wherein the difference between the higherwater content value and the water content value is a measure of theamount of ice water present.
 8. A system for warning of being in acloud, said system comprising: at least one temperature controlled selfheated sensor, a variable power source for the at least one sensor, thevariable power source having an output power responsive to changes incloud water; a processor for averaging the output power of the variablepower source and subtracting a substantially instantaneous measure ofthe output power of the variable power source to determine fluctuationsaround said average; comparing said fluctuations to a thresholdassociated with the at least one sensor indicative of the presence ofcloud water and indicating when the threshold is exceeded.
 9. The systemas claimed in claim 8, wherein the sensor is responsive to changes inliquid water only and the threshold is associated with the presence ofliquid water.
 10. The system as claimed in claim 8, wherein the sensoris responsive to changes in liquid and ice water and the threshold isassociated with the presence of ice water and liquid water.
 11. Thesystem as claimed in claim 8, wherein the fluctuations are determinedusing a frequency content analysis.
 12. A system for determining ameasure of liquid water droplet size based on sensor cooling by cloudwater and stored sensor power ratio and droplet size data, said systemcomprising: a first and second temperature controlled co-located selfheated sensors, the cooling of said sensors being substantiallyindependent of ice water; a self heated compensation sensor oriented tobe substantially independent of cloud water cooling; controller meansfor: keeping each of the first and second sensors at a predeterminedtemperature and each sensor having an input power; keeping thecompensation sensor at a predetermined temperature and having an inputpower; correcting a measured power to each of the first and secondsensors to reflect only heat loss due to cloud water by subtracting fromthe measured power to each of the first and second sensors adjustedcompensation sensor measured values; and forming a ratio of correctedpower of each of the first and second sensors to identify a liquid waterdroplet size based on the stored sensor power ratio and droplet sizedata.
 13. A system for measuring water droplet size in a cloud, whereinthe system comprises: a sensor head comprising: a first heated sensorhaving a resistance characteristic that is temperature dependent,wherein power loss from the sensor is at least essentially not affectedby the presence of ice water in an airflow thereacross; a second heatedsensor having a second resistance characteristic that is temperaturedependent, wherein power loss from the second sensor is at leastessentially not affected by the presence of ice water in the airflowthereacross; a heated compensation sensor being positioned to beresponsive to the airflow thereacross and having a power loss at leastessentially not affected by the presence of liquid water or ice water inthe airflow thereacross; a first controller means associated with eachof the first sensor, second sensor and compensation sensor, formaintaining the temperature of the respective sensors at respectivetemperature values using the respective sensors' temperature dependentresistance characteristic, wherein, for each of the first and secondsensors, the first controller means maintains the sensor at thetemperature value by adjusting a power level fed to the sensor andcalculates the total power required to maintain the sensor at thetemperature value when said sensor is exposed to an airflow comprisingat least one of liquid water and ice water; and a second controllermeans for: receiving power levels from each of the first sensor andsecond sensor; subtracting, from each total power calculation, a drypower component determined from the power required to maintain thecompensation sensor at the compensation sensor's temperature value inthe presence of the airflow comprising at least liquid water, therebyarriving at a wet power value attributable to an incremental powerassociated with maintaining the respective first and second sensors atthe respective temperature values in the presence of at least liquidwater in the airflow; and forming a ratio of the wet power value of thefirst sensor to the wet power value of the second sensor to obtain anestimated value of the measure of water droplet size in the cloud from apredetermined calibrated relationship of wet power ratios to waterdroplet sizes.
 14. A system for determining liquid water content in acloud, wherein the system comprises: a sensor head comprising: a firstheated sensor having a first resistance characteristic that istemperature dependent, wherein the first sensor is at least essentiallynot affected by the presence of ice water in an airflow thereacross; asecond heated sensor having a second resistance characteristic that istemperature dependent, wherein the second sensor is at least essentiallynot affected by the presence of ice water in the airflow thereacross; aheated compensation sensor being positioned to be responsive to theairflow thereacross and at least essentially not affected by thepresence of liquid water or ice water in the airflow thereacross; afirst controller means associated with each of the first sensor, secondsensor and compensation sensor, for maintaining the temperature of therespective sensors at respective temperature values using the respectivesensors' temperature dependent resistance characteristic, wherein, foreach of the first and second sensors the sensor is maintained at thetemperature value by adjusting a power level fed to the sensor; and thetotal power required to maintain the sensor at the temperature valuewhen said sensor is exposed to an airflow comprising at least liquidwater is calculated; and a second controller means for: receiving powerlevels from each of the first sensor and second sensor; subtracting,from each total power calculation, a dry power component determined fromthe power required to maintain the compensation sensor's temperaturevalue in the presence of the airflow thereby arriving at a wet powervalue attributable to an incremental power associated with maintainingthe respective first and second sensors at the respective temperaturevalues in the presence of at least liquid water in the airflow; andcalculating a liquid water content for the first and second sensor wetpower values and selecting a higher LWC as the determined liquid watercontent.
 15. A system for determining liquid water content in a cloud,said liquid water having a determined droplet size, wherein the systemcomprises: a sensor head comprising: a first heated sensor having afirst resistance characteristic that is temperature dependent, whereinthe first sensor is at least essentially not affected by the presence ofice water in an airflow thereacross; a second heated sensor having asecond resistance characteristic that is temperature dependent, whereinthe second sensor is at least essentially not affected by the presenceof ice water in the airflow thereacross; a heated compensation sensorbeing positioned to be responsive to the airflow thereacross and atleast essentially not affected by the presence of liquid water or icewater in the airflow thereacross; a first controller means associatedwith each of the first sensor, second sensor and compensation sensors,each of which for maintaining the temperature of the respective sensorsat respective temperature values using the respective sensors'temperature dependent resistance characteristic, wherein, for each ofthe first and second sensors the means maintains the sensor at thetemperature value by adjusting a power level fed to the sensor andcalculates the total power required to maintain the sensor at thetemperature value when said sensor is exposed to an airflow comprisingat least liquid water; and a second controller means for: receivingpower levels from each of the first sensor and second sensor and havinga stored droplet size verses liquid water content correction curve foreach first and second sensor; subtracting, from each total powercalculation, a dry power component determined from the power required tomaintain the compensation sensor at the compensation sensor'stemperature value in the presence of the airflow thereby arriving at awet power value attributable to an incremental power associated withmaintaining the respective first and second sensors at the respectivetemperature values in the presence of liquid water in the airflow; andcalculating a liquid water content value for the first sensor and secondsensor wet power values, selecting the higher of the calculated liquidwater content values, and correcting the higher value using a dropletsize verses liquid water content correction curve.
 16. A system fordetermining the presence of ice water in an airflow, wherein the systemcomprises: a sensor head comprising: a first heated sensor having afirst resistance characteristic that is temperature dependent, whereinheat from the first sensor is at least essentially not affected by thepresence of ice water in the airflow thereacross; a second heated sensorhaving a second resistance characteristic that is temperature dependent,wherein heat from the second sensor is at least essentially not affectedby the presence of ice water in the airflow thereacross; a third heatedsensor having a third resistance characteristic that is temperaturedependent, wherein heat loss from the third sensor is affected by apresence of ice water in the airflow thereacross; a heated compensationsensor being positioned to be responsive to the airflow thereacross andhaving a heat loss least essentially not affected by the presence ofliquid water or ice water in the airflow thereacross; a first controllermeans associated with each of the first sensor, second sensor, thirdsensor and compensation sensor, each of which for maintaining thetemperature of the respective sensors at respective temperature valuesusing the respective sensors' temperature dependent resistancecharacteristic, wherein, for each of the first, second and thirdsensors, the means maintains the sensor at the temperature value byadjusting a power level fed to the sensor and calculates the total powerrequired to maintain the sensor at the temperature value when saidsensor is exposed to an airflow comprising at least ice water; and asecond controller means for: receiving power levels from each of thefirst sensor and second sensor; subtracting, from each total powercalculation, a dry power component determined from the power required tomaintain the compensation sensor at its temperature value in thepresence of the airflow comprising at least ice water, thereby arrivingat a wet power value attributable to an incremental power associatedwith maintaining the respective first, second and third sensors at therespective temperature values in the presence of at least ice water inthe airflow; and calculating a water content measurement for the firstsensor, second sensor and third sensor wet power values, selecting ahigher water content value from the first and second sensor andsubtracting the higher value from the water content value of the thirdsensor to indicate a presence of ice water.
 17. A method for determininga measure of liquid water droplet size based on sensor cooling by cloudwater and stored sensor power ratio and droplet size data, utilizing asystem comprising a first and second temperature controlled co-locatedself heated sensors, the cooling of said sensors being substantiallyindependent of ice water, a self heated compensation sensor oriented tobe substantially independent of cloud water cooling; a variable powersource coupled to each of the first and second sensors for keeping eachsensor at a predetermined temperature and each variable power sourcehaving an output power; and a variable power source coupled to thecompensation sensor for keeping the sensor at a predeterminedtemperature and having an output power, wherein the method comprises thesteps of: correcting a measured power to each of the first and secondsensors to reflect only heat loss due to cloud water by subtracting fromthe measured power to each of the first and second sensors adjustedcompensation sensor measured values; and forming a ratio of correctedpower of each of the first and second sensors to identify a liquid waterdroplet size based on the stored sensor power ratio and droplet sizedata.
 18. A method of measuring water droplet size in an airflow due topassing through a cloud, utilizing a system comprising a sensor headcomprising a first heated sensor having a resistance characteristic thatis temperature dependent, wherein power loss from the sensor is at leastessentially not affected by the presence of ice water in the airflowthereacross; a second heated sensor having a second resistancecharacteristic that is temperature dependent, wherein power loss fromthe second sensor is at least essentially not affected by the presenceof ice water in the airflow thereacross; a heated compensation sensorbeing positioned to be responsive to the airflow thereacross and havinga power loss at least essentially not affected by the presence of liquidwater or ice water in the airflow thereacross, wherein the methodcomprises the steps of: maintaining the temperature of the respectivesensors at respective temperature values using the respective sensors'temperature dependent resistance characteristic; and wherein, for eachof the first and second sensors, the method comprises the steps ofmaintaining the sensor at the temperature value by adjusting a powerlevel fed to the sensor and calculating the total power required tomaintain the sensor at the temperature value when said sensor is exposedto an airflow comprising at least one of liquid water and ice water;wherein the method further comprises the steps of: receiving powerlevels from each of the first sensor and second sensor; subtracting,from each total power calculation, a dry power component determined fromthe power required to maintain the compensation sensor's temperaturevalue in the presence of the airflow comprising at least liquid water,thereby arriving at a wet power value attributable to an incrementalpower associated with maintaining the respective first and secondsensors at the respective temperature values in the presence of at leastliquid water in the airflow; and forming a ratio of the wet power valueof the first sensor to the wet power value of the second sensor toobtain an estimated value of the measure of water droplet size in thecloud from a predetermined calibrated relationship of wet power ratiosto water droplet sizes.
 19. A method for determining liquid watercontent in an airflow, utilizing a system comprising a sensor headcomprising a first heated sensor having a first resistancecharacteristic that is temperature dependent, wherein the first sensoris at least essentially not affected by the presence of ice water in anairflow thereacross; a second heated sensor having a second resistancecharacteristic that is temperature dependent, wherein the second sensoris at least essentially not affected by the presence of ice water in theairflow thereacross; a heated compensation sensor being positioned to beresponsive to the airflow thereacross and at least essentially notaffected by the presence of liquid water or ice water in the airflowthereacross; wherein the method comprises the steps of: maintaining thetemperature of the respective sensors at respective temperature valuesusing the respective sensors' temperature dependent resistancecharacteristic, maintaining the sensors at the temperature value byadjusting a power level fed to the sensor; calculating the total powerrequired to maintain the sensor at the temperature value when saidsensor is exposed to an airflow comprising at least liquid water;receiving power levels from each of the first sensor and second sensor;subtracting, from each total power calculation, a dry power componentdetermined from the power required to maintain the compensation sensor'stemperature value in the presence of the airflow thereby arriving at awet power value attributable to an incremental power associated withmaintaining the respective first and second sensors at the respectivetemperature values in the presence of at least liquid water in theairflow; and calculating a liquid water content for the first and secondsensor wet power values and selecting a higher LWC as the determinedliquid water content.
 20. A method for determining liquid water contentin an airflow, said liquid water having a determined droplet size,utilizing a system comprising a sensor head comprising a first heatedsensor having a first resistance characteristic that is temperaturedependent, wherein the first sensor is at least essentially not affectedby the presence of ice water in an airflow thereacross; a second heatedsensor having a second resistance characteristic that is temperaturedependent, wherein the second sensor is at least essentially notaffected by the presence of ice water in the airflow thereacross; aheated compensation sensor being positioned to be responsive to theairflow thereacross and at least essentially not affected by thepresence of liquid water or ice water in the airflow thereacross;wherein the method comprises the steps of: maintaining the temperatureof the respective sensors at respective temperature values using therespective sensors' temperature dependent resistance characteristic and,for each of the first and second sensors, maintaining maintains thesensor at the temperature value by adjusting a power level fed to thesensor and calculating the total power required to maintain the sensorat the temperature value when said sensor is exposed to an airflowcomprising at least liquid water; wherein the method further comprisesthe steps of: receiving power levels from each of the first sensor andsecond sensor and having a stored droplet size verses liquid watercontent correction curve for each first and second sensor; subtracting,from each total power calculation, a dry power component determined fromthe power required to maintain the compensation sensor's temperaturevalue in the presence of the airflow thereby arriving at a wet powervalue attributable to an incremental power associated with maintainingthe respective first and second sensors at the respective temperaturevalues in the presence of liquid water in the airflow; and calculating aliquid water content value for the first sensor and second sensor wetpower values, selecting the higher of the calculated liquid watercontent values, and correcting the higher values using a droplet sizeverses liquid water content correction curve.
 21. A method fordetermining the presence of ice water in an airflow, utilizing a systemcomprising a sensor head comprising a first heated sensor having a firstresistance characteristic that is temperature dependent, wherein heatfrom the first sensor is at least essentially not affected by thepresence of ice water in the airflow thereacross; a second heated sensorhaving a second resistance characteristic that is temperature dependent,wherein heat from the second sensor is at least essentially not affectedby the presence of ice water in the airflow thereacross; a third heatedsensor having a third resistance characteristic that is temperaturedependent, wherein heat loss from the third sensor is affected by apresence of ice water in the airflow thereacross; a heated compensationsensor being positioned to be responsive to the airflow thereacross andhaving a heat loss least essentially not affected by the presence ofliquid water or ice water in the airflow thereacross; wherein the methodcomprises the steps of: maintaining the temperature of the respectivesensors at respective temperature values using the respective sensors'temperature dependent resistance and wherein, for each of the first,second and third sensors, maintaining the sensor at the temperaturevalue by adjusting a power level fed to the sensor and calculating thetotal power required to maintain the sensor at the temperature valuewhen said sensor is exposed to an airflow comprising at least ice water;wherein the method further comprises the steps of: receiving powerlevels from each of the first sensor and second sensor; subtracting,from each total power calculation, a dry power component determined fromthe power required to maintain the compensation sensor's temperaturevalue in the presence of the airflow comprising at least ice water,thereby arriving at a wet power value attributable to an incrementalpower associated with maintaining the respective first, second and thirdsensors at the respective temperature values in the presence of at leastice water in the airflow; and calculating a water content measurementfor the first sensor, second sensor and third sensor wet power values,selecting a higher water content value from the first and second sensorand subtracting the higher value from the water content value of thethird sensor to indicate a presence of ice water.
 22. A method ofwarning of being in a cloud using at least one temperature controlledself heated sensor and a variable power source for the at least onesensor having an output power responsive to changes in cloud water,wherein the method comprises the steps of: forming an average of outputpower values of the power source; subtracting a substantiallyinstantaneous measure of the output power of the power source todetermine fluctuations around said average; and comparing saidfluctuations to a threshold associated with the at least one sensorindicative of the presence of cloud water and indicating when thethreshold is exceeded.